Why not send a small postcard with the license key that requires them to sign to get the postcard? Give them an instant key good for 14 days and then make them use the one on the postcard for long term use. Then you'll have your proof (and be pretty close to PayPal's chargeback limit)
The reason you don't want to do that is because they can just reject the postcard. That's why you have to give them needed info on the card. The point of the temp key is so they can get immediate satisfaction.
