The credit card part I can help with. I have a client whose site has online payments and it uses the 'PayFlow Pro' system from Verisign. There is a Perl interface to the library which is very straight forward ($return_hash = pfpro($hash_ref)). I use it under Linux, but I think there are versions for a bunch of different platforms. You do have to do some setup with verisign and your bank to make all of this work, which if I remember correctly is basicly a few forms and a fee. The other thing you'll need is a SSL certificate so that you can run your e-commerce app under SSL. Don't even consider handling credit card info without using SSL on the server.
in reply to interchange for e-commerce?
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"