If you are mailing your wedding invitations after April 10th 2016 and it weighs 3.5oz you will need a $1.10 stamp to mail it 1st-class. This is also the stamp you need if you need to mail a 3 oz letter that is odd-sized/non-machinable. (The price to mail this size and weight letter was previously $1.15. That stamp will still work, it simply costs a few cents more to mail the letter). Be sure to double-check the weight of your invitations with your local USPS post office.
Zazzle offers thousands of designs for wedding-themed $1.10 stamps as well as all other first-class face values that are approved by the USPS.