I know this has been talked about on this blog before, and every bride thinks about how much her invitations will cost and how much will they cost to mail.
But you never know until those bad boys are actually in your hands. Don't assume that just becasue they measure properly for a .42 stamp that they actually will weigh properly for a 1 ounce stamp.
Wait until you have them and then bring them to the post office, have them weigh and measure them - then you will know the proper postage amount. You may even want to bring two, just in case.
{Really beautiful invitation picture from Unique Designs}