I am a member at a museum. I pay a (relatively large) sum of money annually which means myself and a guest can go to exhibitions without paying. I invited a friend to come to an exhibition with me. She said yes please, and then invited a mutual friend to join us. I said to both of them in a group chat that the extra ticket would be £20. They both said that was fine. I booked the tickets and paid the £20. The friend I had originally invited said, “thanks, that’s £6.50 each then” and they both immediately sent me £6.50! I feel so petty quibbling over £7, so I am not going to, but I really feel that I have already paid for the two tickets in the cost of my membership (they are not just “free”) and now I have to pay because my friend invited someone else AND I have ended up 50p more out of pocket because my friend can’t do maths! I will definitely make them buy me coffee and cake on the day, but AIBU to feel a bit put out?