I take the boxes home to have handy, and keep them with my other shipping boxes and bubble pack. I use them where it makes sense. Heavy items in a small package is where the bang for the buck is when going flat rate otherwise it makes sense to ship it the other way.
I am on a GMT400 truck forum. I bought a pair of NOS chevy 1500 door emblems. The flat rate box the guy was gonna use that they fit in without bending was gonna cost $15.00 I asked him to please place the emblems in a mailer tube and get me a price that way. Total cost was $1.75
I shipped a bunch of aviation sheetmetal tools once, tungsten steel bucking bars, drill bits, clecos in a med flat rate box. Packed it up tight. Used all the space. Put their flat rate to the test lol. Had to use duct tape to reinforce the box. About 40 lbs o **** in an $8.00 flat rate box so yeah i got em back lol.