Search for the best upsell app for Shopify and you will find ReConvert, Selleasy, and a dozen others, each built around a different moment in the purchase. That moment matters more than the app. This guide explains the two moments that happen at or after checkout, what each one does to your numbers, and how to decide which you need.
The two moments
An in-checkout upsell appears while the shopper is filling in the checkout, usually next to the order summary, before payment. Accepting it changes the order total before the card is charged. A post-purchase upsell appears after payment is authorised and before the thank-you page; accepting it adds a second charge to the same order without re-entering card details. Shopify supports both through Checkout Extensibility, but they behave very differently.
What in-checkout does to your numbers
The shopper has decided to buy and their intent is at its peak. A relevant one-tap add-on at this moment sees the highest attach rates of any upsell placement, which is why the Upsell block alone accounts for roughly a +6% AOV lift in our merchant data. The risk is distraction: a bad offer, or too many offers, adds friction at the most expensive point in the funnel. The rule is one offer, under a third of the cart value, obviously related to what is in the cart.
What post-purchase does to your numbers
Zero risk to conversion, because the sale is already done. Attach rates are lower, since the shopper's guard is back up and the offer feels like an interruption, but any accepted offer is pure incremental revenue. Post-purchase is the right place for larger or less related items: a second unit at a discount, an extended warranty, a subscription upgrade. Apps like ReConvert specialise here and do it well.
Which products belong where
In checkout: consumables that go with the main item, accessories under $25, gift wrapping, shipping protection, a size-up or bundle of the same product. Post-purchase: a second full-price product at a discount, a warranty, a subscription, anything that needs a sentence of explanation. If you have to explain it, it does not belong next to the pay button.
How to add an in-checkout upsell on Shopify Plus
Install Checkout Ninja and open the checkout editor. Add the Upsell block, or one of its variants: Featured Upsell for a single hero product with an image, Quick Add Upsell for a compact one-tap row, or Promo Banner Upsell for a discount-led offer. Choose products by collection, by tag, or by rule (for example, offer the matching case when the cart contains a phone). Place it above the order summary on desktop and above the pay button on mobile. It is a Deluxe plan block, and it is included in the 3-day trial. It sits alongside the other Conversion Boosters, and our guide to customizing Shopify checkout covers everything else you can add.
Rules that make upsells convert
Do not offer what is already in the cart. Do not offer above the free-shipping threshold if the cart is already past it, and do offer the item that gets them over it if they are just below: the Free Shipping Bar and the Upsell block work together here. Cap at one offer on mobile and two on desktop. Refresh the offer set quarterly; the same add-on for a year goes blind.
Can you run both?
Yes, and the best-performing Plus stores do: a small related add-on in checkout, a larger offer post-purchase. They do not compete because they target different products and different states of mind. If you can only run one, run in-checkout for consumables and accessories, post-purchase for everything else.
Measuring it honestly
Track AOV and conversion rate together for two weeks before and after. An upsell that lifts AOV 5% but drops conversion 2% is a loss on most margins. Checkout Ninja's A/B Testing feature runs the block on half your traffic so you get the answer without guessing.
The one-line version: in-checkout for small, obvious add-ons at peak intent; post-purchase for bigger offers with zero conversion risk; both if you can.