So if I put a note in a hard plastic sleeve, nestled between two pieces of cardboard, and put in an envelope, that would suffice?
Sandwiched between cardboard and placed in a rigid photo mailer: Don't buy them in brick-n-mortar stores, where they sell for $1 to $2 a piece. I got 100 of them for $10.00 on eBay. That's $0.10 a piece. Not expensive.
Agree with the rigid photo mailer. Have used the bubble pack envelopes with hard inserts, but the postmen LOVE to bend that $H!+...
Where do you get your cardboard. I'm getting ready to list about 120 bills and although they're not high grades I still don't want them damaged.