How the hell do you do this?!??
 

How do you allow certain domains access to another site?

eg like plugin providors do?

Just let the refering urls from people who pay through and rest blocked out?

You could use htaccess to limit access from certain domains or you could use a cookie mechanism or some sort of session variable.

how do the content plugin sites do it though

I've seen all three methods used depending on the provider.

ludedude any idea on how to set this up etc can u ICQ me if so?

bump