How do I give the customer access to the membership area after they pay?

hey everyone, I have been asking around on here about a membership script of a site I want to build.

But i just realized that the first thing i should think about is whether it can integrate with my merchant account.

I use fastspring, they do not support recurring billing, but that is okay as I will charge a flat lifetime fee.
But, this where i get confused, How do I give a member access to the site?

Its not like i send them to a download link and they are done.

I asked the support team at fastspring and they said that I would need something that, when an order is placed the order is sent to me and then a username and password is sent back to them automatically??

Does this make sense? It doesnt to me, all the membership sites i have joined, I create the username and password right after i pay, i dont get sent one.

Can anyone help me find what I am looking for, does it even have a name, this username/password automation thingy?
Or is there a membership software/script that will do the, custom username/password all by itself?

Thanks in advance