After reading this, I’m trying to decide whether to try and find rectangle holiday greeting cards, or forego sending cards altogether this year.
If you don’t want to click on the link, the story is as follows (as found on www.usps.com, the website for the United States Postal Service):
Square cards cost 17¢ more.
Because square greeting cards won't flow through standard postal sort machines, we charge a 17¢ nonmachinable surcharge to handle these special cards. So, you'll need a total of 58¢ in postage.
As well, square greeting cards more than 6 1/8" tall are priced as large envelopes. To determine how much postage you need use the USPS.com rate calculator and pick "large envelope" as the shape of your mailing.