PayPal Standard
I think I have gotten a direction with this from PayPal's support. They don't return any electronic indicator (a post return) with their basic payment processing with the payment buttons they generate from their site. They only send an email that would then require manual updates of the client records. According to their support I need to have a full shopping cart setup even though there is only one product at this time.
My biggest confusion at the beginning of talking to thme is that they have a service where they will generate usernames and passwords but that didn't look like it would work at all with the Security Assist files I have setup.
Bottom line is that I think this is handled. I am setting up the eCart and may have a couple of questions as I get that going.