If you are mailing your wedding invitations after January 22nd, 2017 and it weighs 3.5oz you will need a $1.12 stamp to mail it 1st-class. (If your letter is this weight and you're mailing it before 1/22/2017 you'll need a $1.10 stamp. ) This is also the stamp you need if you need to mail a 3 oz letter that is odd-sized/non-machinable. Be sure to double-check the weight of your invitations with your local USPS post office.
Zazzle offers thousands of designs for wedding-themed $1.12 stamps as well as all other first-class face values that are approved by the USPS.