R etailers' deferred interest offers, which let you buy now and pay later with little or no f inance charges, are tempting, especially during the winter holiday buying season. But if you're not careful, these deals can backfire and end up costing you a lot. A new study by the website WalletHub found that 82 percent of consumers don't understand how deferred interest programs work. In part, it blames retailers who bury the potential gotchas in fine print that's difficult to find and read. "Deferred interest financing is like a wolf in sheep's clothing, pairing an enticing offer – something like 'no interest if paid in full' or 'special financing' – with a clause that allows the deal to turn ugly if you make the slightest mistake,"  says the study, which was based on a review of 73 large retailers' financing programs.   Deferred interest financing usually is offered in connection with retailer credit cards, though it also ma