You restock a bestseller. You post about it on Instagram. Maybe three people see it. A week later, half the inventory is still sitting there, and the customers who wanted it most have already bought from someone else.
The problem is not demand. The problem is timing. The customers who wanted that product signed up for an email alert weeks ago, and by the time they open it, the moment has passed. SMS fixes this. Open rates for SMS sit around 98 percent compared to 20 percent for email. And most SMS platforms let you send transactional alerts without a massive marketing budget.
Here is how to build a back-in-stock SMS alert system that triggers the moment inventory hits your warehouse, reaches every interested customer instantly, and creates enough urgency to clear your restocked products in 48 hours.
What You Need
- An e-commerce platform (Shopify, WooCommerce, or BigCommerce)
- An SMS tool (Postscript, Attentive, or SMSBump, all with free tiers or low-cost plans)
- A spreadsheet or simple form to collect phone numbers (if you do not already use an SMS tool)
- 30 minutes
Step 1: Capture Phone Numbers on Your Out-of-Stock Product Pages
When a product goes out of stock, your product page should not just say “Sold Out.” It should offer a back-in-stock alert with a phone number field.
If you use Shopify, you can edit your product page template to show a simple form when inventory reaches zero. Add a phone number input field with a submit button that says “Text me when this is back.” The form saves the phone number to a tag or metafield on the product.
If you use an SMS tool like Postscript or Attentive, they have built-in back-in-stock widgets you can drop into your theme with a single line of code. The widget automatically appears when a product is out of stock and collects phone numbers into a dedicated list.
The key is making the form feel specific and urgent. Instead of “Notify me,” use “Text me the moment this is back in stock.” The wording matters because it sets the expectation that the alert will be fast and personal.
Step 2: Set Up the Inventory Trigger
You need an automation that fires when a product transitions from zero inventory to one or more in stock. This is the trigger that sends the SMS.
In Shopify, use Shopify Flow (available on all plans) to create a workflow:
- Trigger: Inventory quantity changes
- Condition: Previous quantity was 0 AND new quantity is greater than 0
- Action: Send API request to your SMS tool
If you do not have Shopify Flow, use an automation tool like Zapier or Make. Connect your store inventory feed to your SMS platform. Set the trigger to “New inventory level” and filter for products that were previously at zero.
If you use Postscript, their native Shopify integration handles this automatically. You just need to enable the back-in-stock workflow in their dashboard and map it to your waitlist.
Step 3: Write the SMS Message
The SMS copy is what determines whether customers click through and buy. A bad alert sounds like a system notification. A good one sounds like a friend tipping you off.
Here is the template that works:
“[Product name] is back in stock. We only got [quantity] units. Grab yours before they are gone again: [product link]”
Three things make this work:
- The product name reminds them what they wanted without making them think
- The quantity creates instant scarcity. Even if you restocked 50 units, saying “We only got 50” feels limited
- The link goes directly to the product page, not your homepage
Keep it under 160 characters so it fits in a single SMS segment. No emoji. No brand name prefix. Just the alert, the scarcity, and the link.
Step 4: Add a Fallback for Low Restocks
If you only restocked 5 units and you have 200 people on the waitlist, sending all 200 SMS messages at once creates a problem. The first 5 customers buy, the next 195 click through to an out-of-stock page, and you have wasted messages and frustrated customers.
Solve this by batching. If your restock quantity is less than 10 percent of your waitlist size, send to the first batch only. Most SMS platforms let you segment your list and send to a subset. Send to the first 20, wait 2 hours, check if inventory is still available, then send to the next 20.
If you restocked more than 10 percent of your waitlist, send to everyone. The scarcity in the message will drive fast enough conversions that you will not need to worry about overselling.
Step 5: Track and Optimise
After your first back-in-stock SMS send, check three numbers:
- Delivery rate: Should be above 95 percent. If lower, your phone number collection form might be capturing invalid numbers. Add a validation check.
- Click-through rate: Should be above 20 percent. If lower, your message copy needs work. Try a different urgency hook or shorten the link.
- Sell-through rate: What percentage of restocked units sold within 48 hours. This is your headline metric. Anything above 60 percent is strong.
Run this for three restock cycles and you will have a clear picture of how your specific audience responds. Adjust the message timing, the scarcity framing, and the batch sizes based on what actually drives clicks and purchases.
The Bottom Line
A back-in-stock SMS alert is one of the highest-converting automations you can build. It targets customers who have already raised their hand and said “I want this.” It reaches them instantly instead of waiting for an email open. And it creates natural scarcity because restocked inventory is inherently limited.
The setup takes under an hour. The first time it fires, you will wonder why you ever let restocked products sit in your warehouse for a week.
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