Couple of things I’ve been thinking occasionally:
If the sender forgets to write the ID-number, do they get a message about it (from Postcrossing team), when the receiver asks for help?
Or should the receiver mention they needed help for registering the card?
If someone sends me a card with for example fake postage, and I am told not to register it, is the cheater asked to send me another card with real postage? Or what happens to that card in the system? Does it stay in their travelling list, and looks like I didn’t get the card, or is it removed or marked somehow?