Back-in-stock notifications that match the waitlist to the units you actually have
Updated · By Ecomsellertool editorial team
To automate back-in-stock notifications, save each sign-up with the variant, channel and consent record. When stock returns, rules subtract a hold for other channels and low stock, then notify the oldest sign-ups in batches sized to the units left, not the whole list at once. Each batch waits, stock is rechecked, and sends stop when the units fall below your minimum.
- Shopify treats a tracked item as out of stock when its inventory level is at zero or below; by default customers cannot buy it until stock is added (checked October 30, 2026).
- Shopify Flow's Product variant inventory quantity changed trigger passes the variant and its previous quantity, so a workflow can spot the move from zero to stock.
- Klaviyo's back-in-stock settings include a restock minimum, a number of subscribers to notify per restocked item, and a wait between batches, for email and SMS (checked October 30, 2026).
- The FTC's CAN-SPAM guide says commercial email needs an opt-out and a postal address, and opt-outs must be honored within 10 business days.
- The FCC's rule at 47 CFR 64.1200 says consent to calls or texts can be revoked by any reasonable method and must be honored within 10 business days.
The step map: who runs each step
Trigger: A variant's available quantity moves from zero or below to above zero at a location that fulfills online orders, plus every new waitlist sign-up and every inventory change while a release is running.
| Step | What happens | Who runs it |
|---|---|---|
| 1 | A notify-me form on each sold-out variant saves the email or phone number, the variant, the store or channel, the sign-up time, and the consent the person gave for email or SMS. | Rule |
| 2 | When the variant's available quantity rises above zero at a location that fulfills online orders, the workflow reads the units, subtracts the hold for other channels and the low-stock reserve, and gets the releasable units. | Rule |
| 3 | Rules look up the release decision table and work out the first batch: how many sign-ups to notify, oldest first, from the releasable units and your measured conversion rate. | Rule |
| 4 | The batch goes out by each person's chosen channel: email to everyone who signed up by email, SMS only to numbers with a stored SMS consent record, with an opt-out in every message. | Rule |
| 5 | After the wait window, the workflow rechecks the available units, removes buyers and opt-outs from the list, and sizes the next batch, or stops when units fall below the minimum. | Rule |
| 6 | A person approves the first release for a high-demand variant or a large list, and decides whether to hold units back for a launch, a wholesale order or a marketplace. | Person |
| 7 | Orders from notified people are matched back to their batch; conversion per batch, sell-out time and opt-outs are reported, and the waiting demand is passed to purchasing. | Rule |
6 rule steps · 0 AI model steps · 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
- The first release for a high-demand variant, a large list or a launch
- How many units to hold back for other channels, wholesale and low stock
- Message wording, the SMS consent language on the form, and the opt-out text, with your own counsel
- Whether a waitlist should expire, and when old sign-ups are removed
- Turning waiting demand into a purchase order
Which tools can run it
- A back-in-stock app or your email platform's back-in-stock flow, with its restock minimum and batch settings, for one store
- Shopify Flow or Zapier, Make or n8n reading inventory changes and calling your email and SMS tools, with a sheet that holds the hold rules
- Custom code on your accounts, for stock shared with Amazon and Walmart, per-location holds, measured conversion and a hand-off to purchasing
Systems it connects
- Shopify (or another storefront)
- Inventory system or multichannel sync
- Email platform
- SMS platform
- Consent records
- Amazon and Walmart, when stock is shared
- Slack or email for approvals
Build size: Medium
Sending a message is easy; the work is the hold rules for shared stock, the consent records and batch sizing that reads live inventory.
A popular variant sells out. Five hundred people sign up to hear when it is back. Forty units arrive, the app emails all five hundred at once, the forty go in an hour, and 460 people click through to a sold-out page for the second time. Some of them stop signing up. This recipe treats the waitlist as demand to be matched to stock: hold back what other channels need, notify the oldest sign-ups in batches sized to the units that returned, and stop before the list outruns the shelf. It is one page of our automation library.
Three neighbors do different jobs. If you keep selling while out of stock, the customer has already paid, and our pre-order and backorder delay notices recipe applies instead. Our multichannel inventory sync recipe keeps one stock count across channels, and our low-stock alerts and draft purchase orders recipe reorders. This page sits between them.
How many people should you notify when stock comes back?
Size each batch to what you can sell, not to the list. Two formulas do it, and you can build them in a sheet today.
Releasable units = available units − hold for other channels − low-stock reserve − units promised to open wholesale or draft orders
Batch size = releasable units ÷ (conversion rate of a notified sign-up × average units per order), rounded up and capped at the people still waiting
The conversion rate is the share of notified people who order within your wait window. Measure it from your own past releases; until you have one, start low and let the report correct it. The per-unit ratio in the second formula is the same idea as Klaviyo's setting for the number of subscribers to notify for each restocked item: Klaviyo's back-in-stock guide gives the example of 5 subscribers per unit, which notifies the oldest 100 subscribers for a 20-unit restock (checked October 30, 2026). A 20% conversion rate at one unit per order gives the same 5.
An illustrative example, not a client result. A variant has 500 sign-ups and 40 units come back. Five units are held for Amazon through the sync buffer. Releasable units are 35. Past releases show 25% of notified people order within six hours, at 1.1 units per order. The first batch is 35 ÷ 0.275, about 128 people, oldest first. Six hours later, the batch bought less than expected: orders took 18 units, 17 remain, so the next batch is 17 ÷ 0.275, about 62 people. When fewer units remain than your minimum, say 5, sends stop and the waiting demand goes to purchasing.
Which release does each restock get? The decision table
The formula sizes a batch; this table decides whether to release at all, in what order and when to stop. Expected demand is the people waiting × conversion rate × units per order.
| Stock that returned | Waitlist and channel | How many to notify | Order | When to stop |
|---|---|---|---|---|
| Releasable units cover expected demand of the whole list | Store sign-ups, email and SMS | Everyone, in one send | By each person's chosen channel; SMS only with SMS consent | List done |
| Releasable units cover part of expected demand | Store sign-ups, any size | One batch from the formula | Oldest sign-up first, across email and SMS | Units below your minimum, then recheck after each wait window |
| Releasable units below your minimum | Any | Nobody yet | None | Hold for the low-stock reserve; pass the demand to the reorder |
| Stock returned only at a location that does not fulfill online orders | Store sign-ups | Nobody | None | Wait until stock reaches a fulfilling location or is transferred |
| Shared stock with Amazon or Walmart | Store sign-ups; marketplace shoppers are not on your list | Batch from units after the marketplace hold | Oldest first | Marketplace buffer reached or units below minimum |
| Small first restock of a high-demand variant or a launch | Large list | Batch only after a person approves | Oldest first, or a group you choose, such as past customers | A person's call |
| Variant set to keep selling when out of stock | Sign-ups now buying on backorder | Not a back-in-stock release | Not applicable | Use the backorder recipe's notices |
The location row matters on Shopify. Its out-of-stock help page says a tracked item is out of stock when its inventory level is at zero or below, and that an item can show as out of stock online when its fulfilling location has none, even if another location has stock (checked October 30, 2026). A release should read the same location data the storefront reads.
How do you capture sign-ups and keep consent records?
Show the notify-me form only on variants that are really sold out, and save more than an email address. Each record needs the variant, the store or channel, the sign-up time for queue order, and what the person agreed to: an alert about this item by email, by SMS, or both, plus any separate opt-in to marketing. Keep the consent wording and the form version with the record, so you can show later what someone agreed to.
Email rules come from the FTC's CAN-SPAM guide for business. It decides by an email's primary purpose. Commercial email must tell recipients how to opt out, include a valid physical postal address, and honor an opt-out within 10 business days, and the guide puts each violating email at up to $53,088 in penalties (checked October 30, 2026). A back-in-stock alert is not one of the guide's listed transactional or relationship categories. Many brands therefore send every alert with an opt-out and an address; how your messages are classified is a question for your own counsel. This is not legal advice.
For SMS, we state only what the primary source says. The FCC's rule at 47 CFR 64.1200 requires prior express written consent for certain calls that include an advertisement or constitute telemarketing, and says a person can revoke consent to calls or text messages by any reasonable method, honored within a reasonable time not to exceed ten business days (checked October 30, 2026). The workflow sends SMS only to numbers with a stored SMS consent record and treats a reply such as STOP as an opt-out. The consent language on your form is your counsel's call.
What does the workflow do in each step?
The trigger on Shopify is an inventory change. Shopify Flow's Product variant inventory quantity changed trigger runs when a variant's inventory changes through an order, an adjustment or an app, and passes the variant and its previous quantity; for one location you read that location's available quantity (checked October 30, 2026). A change from zero or below to above zero starts a release. Our guide to what Shopify Flow automates covers the trigger in more depth.
Then the release runs as a loop. Rules compute releasable units, look up the decision table, size the batch and send it. After the wait window they reread stock, take buyers and opt-outs off the list and size the next batch. Stock that arrives during a wait waits for the next recheck, so a second restock does not fire a second full send. Every message carries the opt-out, and people who opted out are never in a later batch. There is no AI model in this recipe: every step is a rule or a person.
How do you measure whether the waitlist works?
Report each batch on its own: people notified, orders within the window, conversion rate, units sold, time to sell out, opt-outs and messages that bounced. Conversion per batch is the number that feeds the formula, so the next release is sized from your own data. A falling rate on later batches often means the list has aged; that is a reason to expire old sign-ups.
The waitlist is also a demand signal for purchasing. People waiting × conversion rate × units per order is a rough count of units you could have sold, and it belongs next to the reorder in our low-stock alerts and draft purchase orders recipe and our stockouts page. The low-stock reserve in the first formula is your safety stock, and sell-through rate shows how fast a release clears.
Which tool should run it?
| Level | Fits when | What to watch |
|---|---|---|
| A back-in-stock app or your email platform's flow | One store, stock that is not shared with marketplaces | Check that it has a restock minimum and batch settings; Klaviyo's guide says notifications go to everyone at once by default |
| Shopify Flow, Zapier, Make or n8n calling your email and SMS tools | You want your own hold rules and a sheet of releases | Log every run; consent records must live somewhere you can query |
| Custom code on your accounts | Stock shared with Amazon and Walmart, per-location holds, measured conversion per batch, hand-off to purchasing | Can run on Ecomsellertool Growth OS beside your inventory data |
Who builds back-in-stock notifications?
Ecomsellertool builds workflow automation on your own accounts and systems, and we have built on Amazon's seller APIs since 2017. For back-in-stock releases we agree your hold rules, minimum and consent records first, then connect inventory, the form, email and SMS, and the batch report. Send a project brief below, or read how our workflow automation service works.
The building blocks are work we have shipped:
- Alerts by email and push. For AccountDr, rule-based automation we built for an Amazon account monitoring platform, we optimized push and email notifications for events such as stranded inventory and listing faults.
- Stock tracked with low-stock alerts. For IVCSR, working across Amazon and Walmart, we built inventory management that tracks inventory levels and alerts users to low stock.
Get your back-in-stock release built
Send us your waitlist, your channels and your stock rules
Describe the workflow you want automated. We reply with a written plan: scope, timeline and a fixed price.
Frequently asked questions
How many people should I notify when a product comes back in stock?
Divide the units you can release by the number of units one notified person buys on average, which is your conversion rate times units per order. With 35 releasable units, a 25% conversion rate and 1.1 units per order, notify about 128 people first, then recheck stock and send the next batch.
Should I notify the whole waitlist at once?
Only when the stock that came back covers what the whole list is likely to buy. If 500 people wait for 40 units, notifying everyone sends most of them to a sold-out page again. Batches, oldest sign-ups first, keep the promise for the people who waited longest.
Does Shopify have built-in back-in-stock notifications?
Shopify's out-of-stock help page does not describe a notify-me feature. Most stores use a back-in-stock app or their email platform's back-in-stock flow, and Shopify Flow can start a workflow when a variant's inventory quantity changes.
Do back-in-stock emails need an unsubscribe link?
The FTC's CAN-SPAM guide decides by the email's primary purpose, and a back-in-stock alert is not on its list of transactional categories. Many brands include an opt-out and a postal address in every alert. Ask your own counsel how your messages are classified; this is not legal advice.
Can I send back-in-stock alerts by SMS?
Send SMS only to people whose consent you have recorded for SMS. The FCC's rule says consent to texts can be revoked by any reasonable method and must be honored within 10 business days. Ask your counsel what consent wording your form needs.
When should back-in-stock notifications stop?
Stop when the releasable units fall below your minimum, when the waitlist is used up, or when the product is unpublished. Units below the minimum belong to the low-stock reserve and the reorder, not to another batch of people who may find it sold out.
