Check hotscipts...there definitely are a few of these.
I use one called Payloadz.com and it works with the Paypal IPN.
Customer chooses their item. Makes payment thru Paypal and Payloadz sends them an encrypted download like that only works for a set period of time.
Check it out. It's free to get started and process your first bunch of sales. Once you hit certain levels they charge you accordingly.
http://www.payloadz.com