USPS Stamp Fulfillment Pacakging

Sometimes when I order stamp sheets from the USPS Stamp Fulfillment Center, they ship in a Priority Mail Envelope, which provides decent protection. However, more often lately they are shipped in poly mailers which provide no protection from damage. Sometimes, the cardboard backing the sheets are shrinkwrapped in are bent, but thankfully the stamps themselves so far have arrived fine. However one day, it think I might receive an order from them where the stamps themselves might get bent/creased.

Would adding a few 20c sheets (20x1c stamps) be an inexpensive way to provide extra cardboard protection or would that be a waste of money?

Why bother? The USPS will replace any part of your shipment that may be damaged. Although I will admit, returning damaged stamps can be a bit of a nuisance.

JLLebbert: If you were going to place an order for 26 sheets from the USPS and you knew they were going to be shipped in a poly mailer, would you order all 26 at once or 13 at at time? Any difference if they get bumped around a lot during shipping?

I would probably opt for two shipments of 13 sheets. But only because 26 sheets might not easily fit in my mailbox, tempting the mail carrier to forcibly cram them in. My usual carrier would probably deliver them to my door, but if they arrived on his off day, all bets are off. In any case, damaged panes would be returned.
I stopped collecting modern U.S. at the end of 2020. But over the previous decade, I only had to return damaged stamps on one occasion. And on that occasion, the order consisted of press sheets mailed in a shipping tube. But I usually would order no more than a dozen panes at one time. While I agree that the use of poly mailers was initially of some concern, stamps delivered in said mailers always arrived safely & undamaged. Maybe I was just lucky. But I did buy a lot of sheets between 2010 & 2020. Of more concern to me was when they started shipping panes without the usual cardboard support that normally accompanies each pane. The panes would be shipped in a large envelope (no poly mailer). This was a fairly recent development that did not occur frequently ... and yet, the stamps still arrived undamaged.
If I were you ... and if I started occasionally receiving damaged stamps ... I would probably opt for a P.O. box.