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How do credit card payments work? What ensures the retailer charges the right amount?
When you give your credit card number and authorize a merchant to charge your credit card, the merchant then gives the information to their merchant processor which in turns bills the bank that issued the card (it's a little more complex and it all happens instantly unless the merchant is using the very old fasion imprinting gizmos). It is possible for a merchant to attempt to charge you more than you authorized but if they do they risk a fine ($25-$50 for a chargeback) from their processor, the legitimate portion of the charge as well as increasing the processing fees charged by their processor or even the possibility of loosing their merchant account entirely and being permanently blacklisted by Visa/Mastercard. In short no legitimate business is going to intentionally over charge your credit card.  There really isn't significant risk in using a reputable online retailer's order forms. There is the possibility that their database could be compromised but that risk is lower than the risk of having an employee steal your credit number when you give it to them in person. Besides in the US at least the most you can legally be held liable for is $50 assuming you notice the discrepancy within 60 days of statement the charge appears on and most banks limit liability to $0.  Over the years I have had a number of different credit card numbers stolen and used fraudulently and I have never had to pay any fraudulent charges.