Go ahead, cram that extra pair of jeans and shoes, along with full-size toiletries and other can’t-leave-home-without necessities, into your already bulging suitcase. It’s expandable, isn’t it? Maybe there’s room for a hair dryer, too, and don’t forget that coffee-table book with the pretty pictures.

These days, pack inefficiently and it could cost you as much as $50 if your bag is too hef...