Reuven and Shimon are both visiting Eretz Yisroel from New York. Shimon bought a little too many seforim while in Eretz Yisroel so he doesn’t have any room in his luggage. He asks Reuven if he has any room for some of his seforim and Reuven says yes and agrees to take some of Shimon’s seforim. When Reuven checks in his luggage he is told that they are overweight and he will have to pay $100. Who is chayav to pay for the overweight? On the one hand it is Shimon’s seforim which caused the overweight, but on the other hand Reuven said he has room and agreed to take the seforim?
Reuvein would seem to be responsible for what happened. It was his negligence to not weigh his bags and determine if he had the space for the extra seforim. In addition, assuming the books were not worth 100 dollars, he should not have paid the fee for their delivery. He could have left them there in the airport, they probably would have been placed in the lost and found, Shimon could have retrieved them on his next trip. Shimon would not want to pay more than the value of the seforim for their delivery, if anything Reuvein would have to ask him if he wishes to pay this fee. Hence, SHimon would not be responsible to pay.