Checkout offers work in a similar way to the rows of candy at the checkout in a supermarket. They are impulse buys of simple products at low cost but with a high profit margin.
In your webstore, you don’t have to use a one-for-all selection of products, you can tailor the offers to each order by basing them on the exact combination of products in a customer's shopping cart.
If a customer is buying a pair of shoes, it may make sense to also offer socks as a check-out item, but maybe not a belt. However, if they are buying shoes and a pair of trousers, a belt may actually be a good checkout item to suggest.
It’s always a good idea to choose lower cost products that still have a high profit margin for these suggestions, resembling the supermarket techniques of cheap, available and easy to grab.