First and foremost - congratulations for completing this tutorial (and scrolling down the page to see this message doesn't count!).
Every good tutorial includes a set of exercises for the readers, and we won't make an exception here. If you don't know any more Interchange than we presented in this tutorial, the following tasks might seem too difficult, but you can take a look at the excellent collection of new Interchange documentation :).
Send the customer a receipt by email
Allow customer to specify item quantities
Generate a unique order number for each order
Store each order in a database
Interface with GnuPG or PGP to encrypt credit card numbers in email reports
Organize your products into categories and group lists by category