I know a few people on here have ordered. To make sure you got the right amount of postage, did you just go to the post office and have them weigh an invitation and then go home and order the stamps online? What site did you use (I'm assuming USPS)? Also, did you just use regular Forever stamps for the RSVP cards?
I realize these may be really stupid questions, but it seems easier to just order them and have them in a few days than spend a few days trawling post offices to find enough wedding stamps for the invitations, which seems to happen to a lot of people. I'm sending out about 160 invitations, so I'm figuring that the odds of the two post offices around me having them all are pretty slim.