The Sun Science, Heritage Breeds and Message Monsters series, all from 2021, are not available on the USPS website. What is the likelihood of those becoming in-stock again?
I rarely shop on the USPS web site other than in recent years to order the commemorative yearbook and stamp packet, as most post offices no longer carry them in stock as they did at one time.
Last week (end of March 2022) I went to the post office to buy some stamps for mailing and as the clerk was leafing through the folio to show me what was available, I noticed that they had a stack (a half dozen or more I think) of the Heritage Breeds sheet. Though I had a sheet already I purchased a couple more, recalling this SCF topic from last January.
This trip I was at a larger facility as my more local rural post office was low on inventory of interesting stamps ... I don't use "boring" (to me) definitive's for mailing. I was excited to see the Heritage Breeds sheets but surprised. They were issued May 17, 2021 and my local post office had once told me that for regular postage, any unsold stamp inventory six months after issue date had to be sent back and replaced with more recent issues. As I was at a much larger post office that at one time had been a regional mailing handling facility the protocol may be different.
When I've been looking for particular stamps that are no longer on my local post office's order form they have put out a request to other post offices in the area and I've acquired material that way.