Hey Parcelpostguy, thanks for the response! Sorry for the radio silence. With vacations, etc., I never got back to answering this thread.
You make some good points about supply and demand. I guess it results in low supply, and low-ish demand for the stamps?
Yeah, finding them has not been easy. By carefully watching and waiting, I've managed to find a fair amount of them at pretty good prices. But it has taken a while, and I still have some holes. Having a dated SON cancel is not a demand for me, but more a "nice to have" if I can find it. I can't be too picky with what's available.
Yes, I'm excluding stamps with a First Day of Use cancel. It feels like "cheating", as they're not truly used, which is probably why they're cheaper and easier to find. They seem like more of a philatelic souvenir sold by the Post Office. For high-denomination self-adhesive stamps, I have started "soaking" them off with Ronsonol. Usually, they were used on cardboard boxes or paperboard envelopes, and those materials don't seem "archival-quality". At least cardboard is considered acidic, so it seems like it could eventually damage/discolor the stamps if the stamps are left on it. I imagine paperboard could be acidic as well.
So far, I haven't tried going after the preprinted priority/express mail stamps, probably because I never see them. I remember seeing one cut square recently online, but that's it. The use of priority/express preprinted envelopes is probably even rarer than the use of priority/express self-adhesive stamps, considering how few preprinted regular mail envelopes are used compared to regular self-adhesive stamps. I do see some preprinted regular mail envelopes occasionally, but not often. I do try to keep them as whole covers.
Recently, I have been doing as you said: using the current stamps in mailings to relatives who can return them to me.
Yes, I realize priority/express stamps can be used to pay postage for any heavier packages. In fact, that's where I'm always using them. I never need to mail an envelope by the Priority or Express mail service. Usually, I'm using the stamps on a heavy box being mailed for a birthday/holiday/etc. Problem is, I rarely, if ever, need to mail a box large enough or heavy enough to use an express mail stamp, so I have to look for them elsewhere.
For work, we use FedEx to ship large, heavy boxes to the UK. When the shipments cost $500 or so, I think "Wow, if only we used the USPS, that could be paid with a lot of Express mail stamps.......if only...."