On boom 6 I got a lot with postage due, I just figured it was easier to pay it than send the block back. But we have a really small post office and they are sticklers for details. The Postmaster signaled me out and said my name was mentioned at their meeting and my "squishies" were slowing down the machine! I think the safest rule of thumb is to take them in and have them tell you how much postage is needed. I never had to pay more than 64 cents, well 65 now, except out of the country.
But I can guarantee if you put just one stamp on mine they will hold it and make me pay the extra 20cents. except the F8 they usually can make it with only a first class stamp.
I got some manila envelopes at the 99 cent store 6 X 9 for 99 cents there were 6 per envelope. I also got the cling wrap Brawny brand for 99 cents. I will mail my F8 in a small white envelope with cling wrap but will mail out the finished blocks in the manila envelopes with the clasp removed. Postage is about 80 cents depending on where it is going and the weight.
Isn't it funny how the post offices are supposed to be the same every where but they all bend their rules a little differently.