I asked my scheduler about using an envelope for A shipping and was told it needs to be an unmarked box.
If it's B shipping then, whatever, box or envelope, as long as it's a Ready Post product. I've used both. I asked the Postal Clerk once when doing my regular business about the Ready Post tubes, did they fall into the over size measured category, and she said no. I hope, cause next time B's are bonused I'm mailing the programmable automated sign to my son for his business.
I have a scale, left over from eBay days, and measure my items to make sure they don't go over, or at least to far over, the reimbursed amount. I'm usually just sending surprise packages to my kids out of state so I can wait till they are bonused.
If not, I look at it like the rest of my MSing, better to have some one else pay for it and make a few bucks, than to pay for it myself and make none.