The link is called even for trial accounts when there is no CC processing occuring - it's is called at the very last step of the signup process.
Within our Affiliate Manager we are able to verify sales, therefore trial accounts do get logged, but we wouldn't pay commission until that account has converted to a paid account. There is a lot of trust involved - but that appears to be the norm with Affiliates.
The Affiliate company also test the process before they activated our account, running through a signup to ensure that the correct values were coming back, so I guess they're happy.
PayPal in answer to your other question.
Thanks
Greg