I'd pay $12 because I should only pay for the services that I am using. It's not my fault that the seller has run out of the product that I wanted. Let me put it to those who are willing to pay $18 in another way. Suppose you reserve an economy seat for an airline flight and you arrive at the airport to find that they've overbooked. They do, though, have first-class seats available for several hundred dollars more. Would you be willing to pay for the upgrade or would you expect the airline to provide it to you for no charge? Let's put another twist to that...the airline offers it to you ahead of another potential passenger who is willing to pay full fare for the first-class seat. Will you insist on getting that remaining seat for free or will you pay the difference?