You'll soon have more flexibility when you use your debit card to shop. Starting this summer, new regulations go into effect making it easier to pay with plastic -- without using credit.
Billshrink.com explains how the debit card regulations work, noting that you'll be able to buy as little as you want. Merchants pay fees (and may pass those fees on to you) for credit card purchases, but debit card transactions will be much more attractive. While they can set minimums for credit-based transactions, they'll hopefully be more forgiving about small PIN-based transactions.
While you may save money as a result of these changes, there are tradeoffs. Stores may charge more for credit card transactions, and banks will have to find ways to make up for lost revenue (they won't make as much off credit card swipe fees, so get ready for other fees). In addition, you have more protection with "credit" transactions than with PIN-based debit card purchases.
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