While I was tending the counter at Greer, a young woman inquired if we sold stamps or not. Why yes, we do, was our reply. Then she wanted to know if we had a hundred to sell her. Well, we didn't, but it did prompt us to ask what she was mailing. She pulled out a small card and proceeded to tell us that the UPS Store she went to told her that it would cost additional postage to mail.
Now, I send a lot of mail and I'm pretty familiar with the regulations. I could not figure out why the staff at that mailer would have told her that. I measured it and it was well within the minimum size requirements for First Class mailing. We then directed her to the nearest Post Office. That's the thing, if you have a question, especially about your super-special mail, you really should ask the source. My suggestion is to go on off-hours and, if you can, to less urban locations for the best luck.
The U.S.P.S. blog recently posted this handy little form on their site for quick reference. They also recommend checking with your local P.O. if you have questions. Congratulations and happy wedding mailings, everyone! (Also, the chart is fine reference for any mailing, not just wedding related ones.)