Problem is Paypal probably can't auto generate keys and send them for you, whereas shareit can.
This is true, they can't. They do have some kind of back-end mechanism, so I think you could set up a script on your site and then have PayPal hit the script when a sale goes through, but I don't know any web-scripting languages so I'm not sure how much work that is or how tricky. I think we still handle PayPal transactions manually, although the only ones I generally do are the wrapper ones where people are a little more used to waiting a few hours.