How much more do square mailings cost than standard ones?

Unless they are very large, the shape of the envelope is not relevant. It’s the weight that counts.

You are better off checking at the local post office.

According to this site, Square Envelopes may require an additional $0.17, plus more if they are smaller than 5 inches

The additional $0.17 surcharge seems to be confirmed on the USPS website because they are non-machinable.

With something as important as wedding invitations, bring them to the post office and ask.

Where’s AC when you need him?

Hrm. I apparently stand corrected. My apologies for the false information…

@syz- You guys don’t need me. Fireside has everything under control.

“Today’s high-tech letter sorting technology enables machinable First-Class
Mail letters to be processed at a cost of 44 cents for the first ounce and 17
cents for each additional ounce up to 3½ ounces. Nonmachinable letters
such as square envelopes, envelopes with the address parallel to the shorter dimension, and envelopes containing odd-shaped items cannot be processed on today’s high-speed mail sorters and a 20-cent nonmachinable surcharge is applied for the additional handling costs.”

So…$0.81 an invitation