And I do mean physical pain; I'm waiting for the Advil to kick in.

I'm wondering if any of you fine folks might know the answer to this question b/c I don't quite trust my reading of the USPS's website.

I have a square card -- 5"x5". (Actually I have a couple dozen now.)

Can I use a regular first class stamp on that? Or does it require extra postage (like so many of the square Hallmark cards that draw my eye at CVS)?

Here's the USPS site that gives the min and max for first class mail, and this with the parameters -- but is it really? I might need to go back to bed although I think it was how I slept that has left me in pain this morning.

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https://pe.usps.com/text/dmm100/mailing-domestic.htm?_gl=1*1gptky3*_ga*MTc2MTAyMzk1NS4xNjU2Nzc0ODc1*_ga_3NXP3C8S9V*MTY2MDQwNjA5Ni4zLjAuMTY2MDQwNjA5Ni4w

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EDIT: Gracias, I'm glad I wasn't entirely off my rocker thinking it was going to cost more to mail. I think I'm going to stress how much I want stamps for my bday and Christmas. I'm also glad the Advil kicked in.