think twice... if you allowe the user to download it automatically you'll have problems with fake paypal accounts and those who pay via their parents accounts. i've had this some times and the problem is that the jerk gets your game but you dont get the money.

i am handling it manually. i get an email from paypal.. check wether the money is there, or not and than i send an email to the customer with the download instruction. after the user downloaded it i disable the access.