As Tyson mentioned, our development team is working on a solution, however there is not a timeline as to when that solution is going to be ready to introduce to our system. Most software companies only release updates once every year or so.
As for adding in a way to keep buyers from purchasing more then once for a set amount of time. The fact is adding any new feature to a program requires not just programing the feature but integrating it with our system and then making sure it does not cause errors with other parts of our system. This feature requires our system to ignore orders if information in the order already matches a recent order in our system, this means that when the payment processor sends us the information on the order, we would then need to check the transaction log for your account and verify all the information that Paypal sends us, which will not include a PayPal login or credit card number, so we would be going on information like e-mail address, which most people have more then one of or IP address, which means if more then one legitimate buyer at say a coffee house wants to buy a product, they will not be able to. After getting that setup, we would need to design the interface and then still test it to verify it does not cause new problems with our system and finally release the feature to the general public.