Have you ever made a purchase or reservation using plastic, only to find out the merchant had put a "hold" on some of your funds?

This is called card "blocking," and there are no laws on how much can be blocked, so it's perfectly legal. For example, you might make a rental car reservation with your credit card and find out the rental company put a charge of $500 on your account. Once you pay your bill and settle up, they will credit back the unused portion. It's even more painful if you make the reservation with a debit card, because then $500 in your bank account becomes unavailable. This is common with transactions where you don't know how much you're going to pay when you first present your card. So things like rental cars, hotels and even gasoline.

Another problem is that some companies don't credit back the funds immediately when you pay your bills. They might keep the hold for five business days after you pay. There are no laws regulating this either. So how to handle it? Ask questions and know exactly how much is going to be blocked on your account. And reserve and pay with the same card.

The Federal Trade Commission has some more advice on this.