[ben165]

The random number is just a temporarily solution. The ideal one would be to use the memo field with an unique random string which contains all kind of characters. With this option you would have infinite payments.

The number solution can be improved if you search in the database for "free" numbers. I set the maximum payment time to 12 minutes. In this case, you can offer 100 payments per 12 minutes. I should update the article with this information.