Now sales pros can leave their manual credit-card imprinters at home when they visit customers. Creditel's PowerSwipe combines a cell phone and credit-card reader: Swipe a card through, and the transaction is O.K.'d and processed in seconds. The $25-a-month service is available on five Motorola phones with detachable swipers ($99 to $149) and Nextel cell-phone service. PowerSwipe takes debit cards but not American Express. It may also save you money: Credit-card companies usually charge swipers 1.5% of sales, rather than the 2.5% to 3% for sales that go through handheld sliders, affectionately known as knuckle-busters. Plus, your knuckles will be happy.