A customer visits your product page. The item is sold out. There is no "notify me when available" button. She closes the tab and goes somewhere else. You restock the item two weeks later. She never finds out.
That exchange represents a sale you did not get from a customer who was already willing to buy.
The back-in-stock notification flow captures purchase intent at the moment it exists. A sold-out product page with a "notify me" button collects email addresses from people who specifically want that item. When the inventory level goes from zero to any positive number, Klaviyo or the Back in Stock Shopify app fires an automated email to everyone on that list. These emails convert at exceptionally high rates — frequently higher than any other automated email in a retailer's stack — because the purchase decision was already made. The customer is not being persuaded. She is being informed.
For specialty items and limited-run products, this works particularly well. A gift shop that gets a small quantity of a popular item and sends a back-in-stock email to the 40 people who signed up to be notified will often sell out within hours of restocking. The inventory moves before it even makes it to the shelf.
The new arrival version of this works on the same principle. A customer who opts into "notify me when new [category] arrives" has told you exactly what she is interested in. When a new collection of candles comes in, the people who signed up for candle notifications get an email before the general announcement goes to the full list. Early access to new arrivals is one of the most effective loyalty mechanics in boutique retail because it makes your best customers feel like they are on the inside.
Setting up the back-in-stock flow in Klaviyo requires adding a snippet to your sold-out product page template and configuring the automated email. Shopify has a native back-in-stock notification feature that is simpler to set up. Both connect to your inventory and fire when the product is available again.
The cost of not having this feature is every customer who looked at a sold-out item and left without leaving contact information. You cannot follow up with those people because you do not know who they are. The ones who click "notify me" become recoverable.
Michelle Onizuka is co-founder and Systems Architect at Onizuka Studio. She builds automation and AI systems for small businesses — including independent retail operations across Tampa Bay and beyond.