After May April 10th 2016 if you are mailing your "large envelope" wedding invitations or any other letter the post office deems as a large envelope (sometimes called "Flats") and it weighs 2oz you will need a $1.15 stamp to mail it 1st-class. Letters are considered "large envelopes" or "flats" if they measure greater than 6-1/8 inches high OR 11-1/2 inches long OR 1/4 inch thick. You can determine if your letter is a standard letter or a large envelope by using our postage calculator, but be sure to double-check the weight of your invitations with your local USPS post office.
To summarize, you'll need a $1.15 stamp if your letter:
- weighs 2 ounces AND
- is greater than 6-1/8 in. high OR 11-1/2 in. long OR 1/4 in. thick AND
- has no non-machinable characteristics
Zazzle offers thousands of designs for wedding-themed $1.15 stamps as well as all other first-class face values that are approved by the USPS.