I think that a lot of people aren't familiar with the USPS website https://www.usps.com/ for their label and packaging needs. You gotta sign up, but that shouldn't be a problem for most. Your delivery postperson might even pic up your outgoing at your house, saving you a trip to the PO.
Every conceivable Priority Mail container, whether flat rate or not, is available for FREE, delivered to your residence.
I hardly use MFRB any more because the Regional A box will hold almost as much AND the price is based on weight and ZIP code. My Regional A boxes often ship for anywhere between $6 and $9. A MFRB is way more costly.
Same way with a Regional B versus a similar FRB.
Jus' sayin ...