You can find great deals shopping online. That’s especially true for the holidays. But there is one kind of deal you need to watch for. It could cost you serious money. And you might never know what hit you.

Imagine that you’re shopping online. You find and purchase the items you want. After checkout, on the confirmation page, you see an apparent coupon. It offers you $10 off your next purchase. Not bad. You were planning on doing more shopping, anyway.

It looks like some kind of loyalty bonus from the shopping site. But it isn’t. It’s actually an offer from a third party. You click through and enter your information. You will get a coupon for your next purchase. But your credit card will be charged every month.

These third parties are actually discount shopping clubs. By entering your information, you’ve signed up for a monthly membership. But you’d have a hard time learning that from their Web sites. (Hint: It’s buried deep in the fine print.)

Some of the membership companies that do this are Webloyalty, Affinion Group and Vertrue Inc.

I never heard of them. Maybe I just don’t get around enough. But I’ve heard of the supposedly legitimate sites that work with them. How about Buy.com, Shutterfly.com, Fandango, Priceline and Ticketmaster? I’ve used every one of them. Shame on them!