Ogone is a payment processing system popular in the Netherlands and Belgium.
A comprehensive implementation guide is available here.
The idea is to be able to construct a data structure with all the necessary attributes to send a payment request to the Ogone payment processing system. Expect lots of data declarations and newtypes, but also some simple business logic (generating a SHA from all attributes, etc).
Once implemented, you can try it out yourself with a Haskell web framework. Test accounts for Ogone are free, although they take a day or two to be approved.
This project will make you think on how to structure a program with types.
For example, the Ogone payment system expects a number of required attributes, and a number of optional attributes. What could you do to ensure that the required attributes are always filled in? Perhaps by using smart constructors. Or maybe some requirement checks can be made type errors.
In the end, a POST request is sent to the payment processing system with all of the attributes included. How can we prevent invalid attribute names being generated?
Ogone is able to send a HTTP request to the originating website when one of these states is reached. Accepting and parsing these requests is a good exercise to get acquainted with Haskell web frameworks such as Scotty, Snap and Yesod.
You can also test out the rest of the functionality on a simple Haskell web framework. Bonus points if you include some kind of form redirect generation functionality.