I took one to the PO and had them weigh it, and they wanted $1.21, until I challenged it and asked for the template. If it fits through the template and weighs less than 2 oz, it goes for $.71. So, I use the 6" X 9" envelopes and I package the candies in two stacks the length of the card and wrap in plastic wrap. Getting the envelope thin enough to pass through the template is the trick, because otherwise it won't run through their machinery. If it's too fat, then it goes for the higher price because it has to be handled by hand.
Postal workers make mistakes too, so that could be part of the reason that there is postage due.