This Shopify app is to show different offers after the order is placed & confirmed, but before the thank you page, so there is no fear of loosing sales.
Customer can redeem the offer and the same order will be updated with new payments calculation. With this app, you can offer multiple offers based on accept or decline initial offer by customer.
App comes with various eligibility rules, which decides when to display offer and when it should not. for example, Based on Items count in the cart, amount of items added in the cart, product added in the cart, etc.
Here is the example of Post Purchase Offer page,