The sky is the limit :). Are you saying you want to process the payment inside of access? Or just keep track of the payments/billing side and run charges through something else?

Access runs my whole business... including what you just mentioned. See Membership Database.

And yes, I use PayPal and their PayFlow service so that Access can act as a credit card processor. You just submit an HTTPS request to their server and get a result back. I don't have any lessons on this because not many people have asked about it, but they have documentation available on their site.