Thanks for the reply, Jerome.
We have serials from db, and unfortunately for most our products refill will not work, since it's almost impossible to refill same product all the time because of unavailability (we are selling also 3rd party serials, which could be unavailable). And in case we will be forced to make a refund because of lack of the stock, we will have to pay additional fees to the payment system for the refund, which could be really high. Maybe it's still possible to switch on the reservation somehow? Otherwise, soon we have to expect consequences I afraid.