If you are thinking to buy those hip square greeting cards for this holiday season, you may want to think again, according to newsobserver.com.

A rectangular envelope, 5 1/2 by 8 1/2, gets the usual 41-cent first-class stamp. But one of those trendy square 6 1/2-inch greeting cards will cost you 58 cents. If your list is long, it adds up.

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So what size do the machines like?

Carl A. Walton, a spokesman for the U.S. Postal Service, said that would be any envelope that’s 11 1/2 inches long and 6 1/8 inches high — or smaller, as long as it’s not square. Anything outside of those dimensions could be considered “nonmachinable” and charged extra postage.

A surcharge is also charged for envelopes with clasps, strings, button or other closures; and letters that are rigid or enclosed in plastic material. To learn more about rates and extra services, go to www.usps.com/rates /extra-services-rates.htm.