Automate low-stock alerts and draft purchase orders your team approves
Updated · By Ecomsellertool editorial team
To automate low-stock alerts and draft purchase orders, pull stock every day from Shopify, Amazon FBA and your 3PL, add open POs, and compare each SKU with its reorder point: average daily units sold times supplier lead time, plus safety stock. When a SKU drops to that line, rules build one draft PO per supplier, rounded to case packs and minimums, and a person approves it before it is sent.
- Shopify Flow's inventory quantity changed trigger runs when orders, adjustments or apps change a variant's stock, and a Shopify template emails you about low inventory (checked October 3, 2026).
- Shopify Flow is a free app on the Basic, Grow, Advanced and Plus plans.
- Shopify admin purchase orders are saved as Draft or marked Ordered, can be saved as a PDF for the supplier, and are received through a linked inventory transfer.
- Amazon's Restock Inventory report, available through the Selling Partner API, recommends products to restock, suggested order quantities and reorder dates for FBA stock.
- QuickBooks Online includes purchase orders only on the Plus and Advanced plans, and a PO can be emailed to the vendor with Save and send.
- In this recipe fixed rules set every quantity, an AI model only adds notes, and a person approves each PO before a supplier sees it.
The step map: who runs each step
Trigger: A daily schedule after your stock syncs, plus an instant alert when a Shopify variant's stock changes and falls below its reorder point.
| Step | What happens | Who runs it |
|---|---|---|
| 1 | Every morning the workflow pulls available, inbound and committed units per SKU from Shopify locations, Amazon FBA and the 3PL's feed, plus the units on open supplier POs. | Rule |
| 2 | It computes each SKU's average daily units sold over the last 30 in-stock days and its inventory position: units in every location, plus inbound and open POs, minus units committed to orders. | Rule |
| 3 | Each position is compared with the SKU's reorder point; SKUs at or below it raise a low-stock alert in Slack or email with the days of cover left. | Rule |
| 4 | Triggered SKUs are grouped by supplier, other SKUs from that supplier within 14 days of their own reorder point join the order, and each quantity is set up to the target days of cover, rounded to the case pack and the minimum order. | Rule |
| 5 | An AI model reads recent sales and notes for each line, flags demand that looks unusual, such as a one-off bulk order, a promotion or days out of stock, and writes a one-line reason for the approver. It never changes a quantity. | AI model |
| 6 | One draft purchase order per supplier is created in your purchasing system, with the worksheet behind every line attached and any flags at the top. | Rule |
| 7 | A buyer reviews the draft, edits quantities, accepts or rejects minimum-order trade-offs and approves it. | Person |
| 8 | Only after approval is the PO sent to the supplier and logged as open, so the next run counts it in the inventory position. | Rule |
6 rule steps · 1 AI model step · 1 person step. A rule follows fixed logic you approve; an AI model handles messy input and its output is checked by rules; a person makes the calls listed below.
What stays with a person
- Every purchase order before it reaches a supplier, with the worksheet for each line
- Minimum orders that push a SKU far past its target days of cover
- Lines the model flags for unusual demand, and new or discontinued SKUs
- Lead-time, case-pack and price changes in the supplier settings
- Orders above the spending limit your team sets
Which tools can run it
- Shopify Flow's inventory trigger for a low-stock email, when stock lives in one Shopify store
- Zapier, Make or n8n reading a reorder-settings sheet, for a few suppliers and one or two stock sources
- Custom code on your accounts, for Amazon FBA plus Shopify plus a 3PL, many suppliers and draft POs in your accounting system or ERP
Systems it connects
- Shopify inventory
- Amazon FBA (Selling Partner API)
- 3PL inventory feed
- Supplier settings: lead times, case packs, minimums
- QuickBooks Online, Xero or an ERP
- Slack or email
Build size: Medium
The math is simple; the work is one trustworthy stock position across channels and clean supplier settings for every SKU.
Most low-stock alerts fire too late. A warning at 20 units means nothing when the supplier needs 45 days and you sell 12 a day. The fix is not a louder alert; it is an alert set at each SKU's reorder point, with the purchase order already drafted when it fires. This recipe does both. It reads stock from Shopify, Amazon FBA and your 3PL, works out who to reorder from and how much, and puts a draft PO in front of a buyer. Nothing reaches a supplier until that person approves. It is one page of our automation library.
How do you work out when to reorder and how much?
Start with the worksheet, because every rule in the workflow comes from it. The reorder point is the inventory position at which a new order has to go out so stock lands before the safety stock is used up.
Reorder point = average daily units sold × supplier lead time in days + safety stock
Two definitions keep it honest. Average daily units counts only days the SKU was in stock, so a week out of stock does not read as a week of no demand. The inventory position is units available in every location, plus units inbound to FBA, plus units on open supplier POs, minus units already committed to orders. For example, a tumbler with 260 units available at FBA, 40 inbound, 90 in the Shopify warehouse and 30 at the 3PL, with 10 committed, has a position of 410.
The quantity rule sits beside it. The order brings the SKU up to a target days of cover after the stock lands:
Order quantity = average daily units × (lead time + target days of cover) + safety stock − inventory position, rounded up to the case pack, then raised to the minimum order
Here is the worksheet for three SKUs from an illustrative brand that sells on Shopify and through FBA. The numbers are examples, not client data.
| SKU | Supplier | Daily units | Lead time | Safety stock | Reorder point | Position | Status |
|---|---|---|---|---|---|---|---|
| Tumbler 20 oz | Supplier A | 12 | 45 days | 120 | 12 × 45 + 120 = 660 | 410 | Below: triggers a PO |
| Lid set | Supplier A | 5 | 45 days | 50 | 5 × 45 + 50 = 275 | 300 | Above, but within 14 days of it: rides along |
| Bottle brush | Supplier B | 3 | 14 days | 15 | 3 × 14 + 15 = 57 | 48 | Below: triggers a PO |
Supplier A ships by sea, so its target cover is 60 days; Supplier B is domestic, at 45 days. The quantities follow the rule step by step:
| SKU | Order-up-to level | Need | Case pack | Rounded | Minimum order | Draft quantity |
|---|---|---|---|---|---|---|
| Tumbler 20 oz | 12 × (45 + 60) + 120 = 1,380 | 1,380 − 410 = 970 | 24 | 41 cases = 984 | 500 per SKU | 984 |
| Lid set | 5 × (45 + 60) + 50 = 575 | 575 − 300 = 275 | 50 | 6 cases = 300 | 200 per SKU | 300 |
| Bottle brush | 3 × (14 + 45) + 15 = 192 | 192 − 48 = 144 | 12 | 12 cases = 144 | 200 per SKU | 17 cases = 204, flagged |
The result is two drafts: Supplier A gets one PO for 1,284 units across two lines, and Supplier B gets one for 204 units. The brush is flagged because the minimum order adds 60 units, 42% more than it needs, and lifts its position to 84 days of sales against the 64 days its order-up-to level allows. A person decides whether to accept that or ask the supplier for less. To test one of your own SKUs, our reorder point calculator runs the reorder point formula.
When does the workflow create a draft PO?
The worksheet only becomes automation once the rules for acting on it are written down. These are the rules we start from, and each one is a setting your team can change:
- Run once a day, after every stock source has synced, so the position is complete. Shopify can also send an instant alert between runs.
- Trigger a SKU when its inventory position is at or below its reorder point. Open POs count, so the same shortfall never triggers twice.
- Group triggered SKUs by supplier, and add any other SKU from that supplier within 14 days of its own reorder point, so you place one order instead of three in a fortnight.
- Keep one open draft per supplier. A later run updates that draft rather than creating a second one.
- Skip SKUs marked discontinued or on hold, and flag any SKU with no lead time, case pack or minimum on file.
- Flag a line when the minimum order adds more than 25% to the computed need, when daily sales moved more than half against the previous 30 days, or when the PO total crosses your approval limit.
- Never send. The draft waits for a person.
The supplier settings behind these rules are a table of their own: lead time, case pack, minimum order and target cover per supplier and SKU. Most of the setup time goes into that table, because it often lives in a buyer's head or an old email thread.
Where does the stock data come from?
From each system's own documented route, read on a schedule. Shopify's InventoryLevel object tracks available, on-hand, incoming and committed quantities for each item at each location (checked October 3, 2026). On the Amazon side, the FBA Inventory API separates fulfillable, inbound, reserved, unfulfillable and researching units, and the workflow counts only what can sell or is on its way.
| Source | What the workflow reads | Used for |
|---|---|---|
| Shopify | Available, incoming and committed units per location | Position and Shopify sales |
| Amazon FBA | Fulfillable and inbound units through the Selling Partner API; Amazon's restock recommendation | Position, FBA sales and a cross-check |
| 3PL | Units on hand and in receiving, from its API or a daily file | Position |
| Supplier settings | Lead time, case pack, minimum order, target cover | The worksheet |
| Purchasing system | Open POs and their expected dates | Position |
Amazon's own suggestion is worth reading, but not following blindly. Amazon's FBA reports documentation describes the Restock Inventory report as recommendations on products to restock, suggested order quantities and reorder dates, and says it can only be requested (checked October 3, 2026). It plans for FBA alone. When you also sell on Shopify or wholesale, the supplier order has to cover every channel, so the workflow sizes the PO itself and flags SKUs where Amazon's number differs sharply from the worksheet.
Which tool should run it?
The simplest one that sees all your stock. If everything sits in one Shopify store, start with Shopify's own tools. Shopify Flow is a free app on the Basic, Grow, Advanced and Plus plans, and its Product variant inventory quantity changed trigger starts a workflow whenever orders, manual adjustments or apps change a variant's stock. Shopify offers a template that emails you when a variant drops below a threshold you set (checked October 3, 2026).
| Level | Fits when | What to know |
|---|---|---|
| Shopify Flow | One store, stock in Shopify locations only | A threshold per variant, not a reorder point that moves with sales; no FBA or 3PL stock |
| Zapier, Make or n8n with a settings sheet | A few suppliers, one or two stock sources | The worksheet lives in a spreadsheet; log every run so a silent failure shows up |
| Custom code on your accounts | FBA, Shopify and a 3PL together, many suppliers, drafts in your accounting system or ERP | Can run on Ecomsellertool Growth OS beside your other inventory flows |
Where the draft lands depends on where your team already raises POs. In Shopify's admin, purchase orders can be saved as a draft or marked Ordered once the supplier confirms, and receiving is tracked on a linked inventory transfer. Shopify's guide to creating purchase orders says you can save a PO as a PDF to send to the supplier. QuickBooks Online's purchase orders are available only on the Plus and Advanced plans and can be emailed with Save and send. Xero's purchase order page says you can email POs to suppliers and copy an approved order into a bill (all checked October 3, 2026).
What should stay with a person?
The purchase order itself. It spends money and commits you to a supplier, so the workflow drafts and a buyer decides. The draft arrives with the worksheet for every line, the model's note on anything unusual and Amazon's suggestion beside your own number, so approving a clean PO takes a minute. The real judgment calls are the flagged ones: a minimum order that buys three months of a slow SKU, a spike that came from one wholesale order, a supplier whose lead time slipped. Paying the supplier stays with your finance team, as in our payouts to QuickBooks or Xero recipe.
If your demand moves with seasons and promotions, or stock has to be split between FBA, Walmart and a 3PL, a fixed average will not keep up. That is the job of our inventory and replenishment agent, which forecasts demand and drafts FBA shipments as well. Our replenishment page covers the multi-location plan.
Who builds this for ecommerce brands?
Ecomsellertool builds workflow automation on your own accounts and systems. We have built software on Amazon's seller APIs since 2017 and shipped 50+ tools for 100+ teams. For purchasing we map your stock sources, suppliers and approval rules first, then build the worksheet, the alerts and the draft POs. Send a project brief below, or read how our workflow automation service works.
The building blocks are work we have shipped:
- Amazon inventory pulled into an operations system. For EzBookPrep, a platform for buying, prepping and selling books, we built vendor management, Amazon inbound and outbound shipment flows, and inventory tracking fed by Amazon's inventory and order reports.
- Alerts on Amazon account events. For AccountDr we built push and email notifications for events such as stranded inventory and suppressed Buy Box, rule-based automation of the same kind as the low-stock alert here.
Orders that come in from wholesale buyers by email are the other side of the same stock; see our B2B order entry recipe.
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Frequently asked questions
How do I set up low-stock alerts in Shopify?
Shopify Flow, a free app on the Basic, Grow, Advanced and Plus plans, has a trigger that fires when a variant's inventory changes and a template that emails you when stock drops below a threshold you set. That covers one store. If you also sell through Amazon FBA or hold stock at a 3PL, the alert needs the stock from those places too, which takes a workflow tool or custom code.
How do I calculate when to reorder a product?
Multiply the average units you sell a day by the supplier's lead time in days, then add safety stock. When the units you have everywhere, plus units already inbound or on open purchase orders, fall to that number, it is time to reorder. Count sales only from days the product was in stock, or the average will be too low.
Can a purchase order be sent to a supplier automatically?
It can, but we recommend against it. A purchase order spends money and commits you to a supplier, so in this recipe the workflow drafts it and a person approves it. Once a buyer approves, the send itself can be automatic, for example through QuickBooks Online's Save and send or Xero's email to suppliers.
How much should I order when a SKU hits its reorder point?
Enough to bring the SKU up to a target number of days of cover after the order lands, minus what you already have and have on order. Then round up to the supplier's case pack and raise it to their minimum order. If the minimum pushes cover far past your target, a person should decide whether to accept it.
Should I use Amazon's restock recommendations instead?
Use them as a cross-check. Amazon's Restock Inventory report suggests quantities and reorder dates for FBA stock, but it plans for Amazon only. If you also sell on Shopify, wholesale or another marketplace, the supplier order has to cover every channel, so the workflow sizes the PO itself and flags SKUs where Amazon's suggestion differs sharply.
Do I need an inventory app or a custom workflow?
If you sell from one Shopify store with a few suppliers, an inventory app or Shopify Flow plus a spreadsheet is often the better choice. A custom workflow earns its cost when stock sits in FBA, a 3PL and your own warehouse, suppliers have different minimums and lead times, and the PO has to land in your accounting system or ERP.
