It is oldest order in the queue gets to pick which card you want up to the time its your turn.
Like walking into a shop, picking a ticket number paying a deposit and letting them know your preferred option.
All the time you are waiting to be served, you can can swap and change your mind. Even if the person behind you has paid for their preferred card already, you can choose to take the last one off the shelf.
I understand the idea, in a normal in store situation, but these were specific pre-orders. Surely the 10th order for a specific card should get the 10th one on the shelf - or 10th through the door.
You would not like paying for a curry, going to pick it up am to find out that that actually they just gave your food away. The guy before you wanted a vindaloo but they had none left as the footie team had just cleared them out, so gave him your madras.
I think it will help with reducing cancellations - at least until people found out this was the case.
If you got in early with an order that is being held up to to shipping issue or low stock, you can swap and get a card sooner so you wont cancel,
if you know you ordered late on the 17th or a few days after you already know that you have a long wait ahead and so would either wait it out or cancel.