The thing is that we don't know why it occurs. But as I iterated in my original statement, the order numbers get repeated once in a while. Very few times in fact, but it does happen. I think I figured out the problem with the charge of $1 for downloadables, so no need to worry about that for now. My worst problem is implementing Paypal Subscriptions for which I can't get support from, even from Paypal. There is documentation here https://developer.paypal.com/webapps/developer/docs/classic/paypal-payments-pro/integration-guide/WPRecurringPayments/ and then going down to Recurring Payments With Direct Payment. I would like Paid support for that if you can help me.