1. Navigate to your Shopify Admin > Settings > Checkout


2. Scroll down to the post-purchase page section. Choose UpsellPlus Checkout Offers.

3. Click 'Save'!
Here's a quick gif of how to do this:

That’s it. Once saved, the offer will appear for your customers.
Shopify has some limitations and considerations for the post-purchase page, you can read about them here.
The main restrictions are that a post-purchase offer won’t work with:
- Shop Pay
- Express Payment methods (Apple Pay, Amazon)
- Paypal does work, but you’ll have to enable it