You know the drill – you’re at the checkout counter, buying a new shirt, and the clerk asks for your zip code. Seems harmless enough – they probably just want to know where their customers come from. But experts warn that sharing your zip code could set you up to be targeted by telemarketers and junk mail.

Retail analysts say that it’s become the norm for retailers to ask for your zip code. And stores do use that info for non-sinister purposes – like to help them figure out where to open new locations, based on where customers live. A zip code can also help them offer more personalized merchandise, and coupons. For example, if sales show that a lot of shoppers with Boston zip codes are buying electric blankets, then home goods retailers can send Boston residents discounts for electric blankets.

But here’s where the problem lies, a growing number of companies pair your zip code with the name on your credit card – which gives them instant access to your mailing address and phone number. And that information is a potential goldmine, because stores can send you their catalogs – or sell your profile to telemarketers. And you may get flooded with promotional phone calls and mailers.

So, think twice before giving out your zip code to a retailer! The only time it’s necessary is at a gas station when you pay with your credit card. You’ll sometimes need to punch in your zip for security purposes, but the info isn’t kept by the gas station.