There are several ways, but this is a more complex question than it may seem like it is.
You will have to deal with bad email addresses, deciding which information to send, showing delivery times, etc. You also have to be very careful with credit card information -- if you have not taken the proper security measures (by enabling PGP credit card encryption or using CyberCash) you might just mail them their own unencrypted credit card number!
This is supported in Minivend via a UserTag, [email ...]. See the ``simple'' and ``basic'' demos.