Klaviyo back in stock: a signal you cannot afford to waste
A Klaviyo back in stock flow is one of the most direct ways to turn demand into sales. When a customer signs up for a sold-out product, they tell you exactly what they want and when they want to buy it. That is a purchase signal in its purest form, and at Mercive we see many brands leave it unused because the setup never gets prioritized.
Unlike an ordinary campaign, a back in stock message reaches a customer who has already declared their intent. The conversion rate on these flows is therefore typically far higher than the average newsletter. The point is simple: if you bring Shopify and Klaviyo together around sold-out products, you sell items you would otherwise have lost to a competitor.
How the flow works technically
When a visitor clicks a "Notify me" button on a sold-out variant in your Shopify store, Klaviyo tracks a back in stock event on the customer's profile. That event is the trigger for the entire flow. In practice, Klaviyo listens to the inventory level on the variant, and when the count rises above zero again, the message is sent automatically to everyone who signed up for that specific variant.
The crucial detail is the variant level. If a customer has signed up for a t-shirt in size M, the message should only trigger when M comes back in stock, not when S or L does. A correct setup of the integration between Klaviyo and Shopify ensures that product data and inventory status are synced precisely, so no customer receives a message about an item that is still sold out in their size.
How we build a flow that converts
We rarely recommend settling for a single message. The first email should be sent quickly after the item is back, ideally right away, because purchase intent drops with every hour that passes. It should be short, show the product clearly and lead straight to the product page with a visible buy-now call to action.
After that, it makes sense to add a follow-up after a suitable interval to those who opened but did not buy. Here you can add a light scarcity signal, for example that stock is limited, as long as it is true. For brands with a high sales pace, the flow should also include a check that stops the message if the item is already sold out again, so you do not send the customer to an empty product page. That is the kind of logic that separates a flow that works from one that creates irritation.
What is Klaviyo, and what does it cost?
Klaviyo is an email and SMS marketing platform built for e-commerce, and it integrates deeply with Shopify. It pulls in order, customer and behavioral data so you can automate messages based on real behavior, including back in stock events, cart abandonment and repeat purchases.
What Klaviyo costs depends on the number of contacts and which channels you use. The platform has a free tier for smaller lists, and the price rises in steps with list size and whether you add SMS. For serious brands, a well-built back in stock flow typically pays for itself quickly, because it captures sales on items where demand has already been proven.
Mercive's approach to back in stock
We never treat back in stock as an isolated email. It is part of an overall CRM strategy where the flow connects to the rest of your setup, your brand universe and your inventory management. A sign-up button on the product page is only worth having if it is placed correctly in your theme and signals the right thing in relation to your brand.
When we set up a flow, we start with the integration and data quality, then the trigger logic at the variant level, and finally the content and timing itself. That order ensures the messages actually trigger correctly before we optimize copy and design. That is the difference between a flow that looks nice in a presentation and one that delivers measurable revenue every month.
How Mercive can help
Want to take this further? Read more here: