As Mike says, this is real custom stuff. It involves some sort of commerce
tracking as you not only have to generate gift certificates with unique
numbers, you need to track their usage and prevent them from being used
again (as well as track who used them just in case some jerk uses a bogus
one).
You're probably better off focusing on a commerce solution that will meet
your needs and is acceptable by your web host (since you will need a
database of some sort and the application will use a scripting technology
that must be supported by your host to work).
