Amazon tells customers that renting textbooks instead of buying them can save up to 80 percent off the purchase price. What Amelia SanFilippo, a thrifty college freshman, wasn’t expecting was that the online retailer would withdraw nearly $4,000 from her father’s checking account because she was a few days late returning the book. In February, the 19-year-old SanFilippo, a cognitive science major, used her father’s debit card to rent Cultural Anthropology: A Toolkit for a Global Age for the spring semester at the University of Delaware. Cost: $62.70. The book was due back June 24. She had asked her father to mail it for her. But it slipped his mind while he was packing for a week-long trip. On June 28, she received an email from Amazon with the subject line: “Your Amazon.com rental has been purchased. The item is now yours to keep.” Cost: $3,800.60 — more than 30 times the price of the textbook. When she called her father in a panic the next day, he told her not to worry. They’d never actually charge him that amount, and he’d be home in a day and would return the book then. Imagine his surprise when he checked his balance at an ATM and found it almost $4,000 down. The forgetful father got the book back to Amazon and they refunded his money.