First, if anyone else is doing calligraphy or hand writing your invite envelopes and you're worried about them smearing or getting trashed in the mail and crappy weather, get Micro Glaze. I got mine on Amazon but I think they sell it at Walmart. It's a clear glaze that you rub on the lettering and it's makes it water/smear proof. We tested it and the water just beads on the paper.
Second, I sent out my invites this morning (yes, I'm about 6 months out but all of our guests are coming from different states and I am deviously trying to get our wedding in the forefront of their minds during tax return time lol) and I stopped at my local post office as soon as they opened. The sweetest lady hand cancelled every single one of my envelopes and didn't charge me for the extra stamp I needed for my invite going to the UK. FH wanted wax seals on the back of the envelopes lol so I wanted to be sure they didn't have to go through a machine. YAY!!!!!