ecommerce e-commrce module

I'm trying to write a module for ecommerce which adds a step to the ordering process where a user can add special instructions. I've got that bit working, but I can't figure out how to save the data entered with the order, and present it later on. Can I tack it on the transaction ($txn) and expect it to be saved in the DB?

What would be the best way to do this?

Thanks for the help!