Okay...I've read it before on here from others...the tip is: Take a fully finished/completed invitation, envelope and all to the Post Office. Do IT.
Picked up a completed invitation...officially weighed it in my hand and declared to FW: This has GOT to be $0.45. It barely weighs anything. Then I remembered what people wrote..so we walked to our neighborhood post office to get postage stamps and have it weighed officially. I told the USPS smugly, " 45 cents, right?". "No sir...$0.65. Do want stamps with that?
. So we promptly bought the $0.65 stamps along with 45cent ones for the RSVP envelopes and went on our marry (pun intended) way. Moral is: WEIGH your invites!! 