What happens if my upsell product is out of stock for post-purchase upsells?

Will my customer still see the upsell. Will they be able to add this to their purchase?

Currently, we will still show OOS products.

If shoppers accept OOS products on post-purchase upsell, by default, these products won't be added and charged in the final purchase. Whilst it does not hurt the customer, we believe such an experience might degrade a customer's trust in the portal.

If you are intending to fulfill backorders of potential OOS products, we would ask you to ensure you enabled the option on products "continue selling your products

We would recommend you consider turning on your shopify setting to have the product selling even when OOS.

see how to set that up on Shopify https://community.shopify.com/c/technical-q-a/set-continue-selling-when-out-of-stock-for-all-my-products/td-p/682098