As of this writing, any envelope that is larger than 6 1/8″ x 11 1/2″ will require additional postage. Keep this in mind when selecting your wedding invitation size and corresponding envelope. Also note that some non-standard envelopes, such as square envelopes require additional postage. Additional postage may also be required if your envelope is too thick, rigid, or not flat. We encourage you to check out the postage regulations yourself in case there have been any changes at USPS.com.