Once you're logged in to your Shopify Admin, open Recheck. Then click on Upsell Offers in the left navigation menu.
Create upsell offer
Click Create upsell in the top right corner
Enter a name for the upsell offer.
Select the triggers based on which you want your upsell offers to be shown.
Select which upsell products that you want to offer in the post purchase upsell.
If you want to offer a discount, choose the discount type and set your discount value.
Activate the offer by clicking the toggle, then click Save
Edit priority order
If an order matches the conditions for multiple upsell offers, Recheck shows upsell offers based on the priority order set by you.
If you want to change the priority order:
Click Edit Priority Order
Drag to rearrange the order of the upsell offers.
Click Save.