I use Printfile polypropylene sleeves pretty much. Although I used to use some of their other sleeves that had a folded edge seam. I use polyethylene bags for ephemera storage.
I think polyester = Mylar. It is very archival. I have some old Mylar from the 60's / 70's and it is crystal clear.
Problem with polyester film leader is it is a dust magnet for static electricity. I prefer acetate film leader on the movie stock. But acetate can suffer from vinegar syndrom
Some plastics are terrible for archival durability. They yellow over a short time. I got lots of test photos, but can't post em here. Google messed me up when they removed their easy link feature for copyright issues.
For boxes I settled on Hollinger metal edge boxes. I used to use Printfile boxes but had problems with supply and many of them stink like hell. They must use animal glue or something - they reek. Tried the new school plastic archival boxes and hated em. They were dust magnets and warped. Very happy with Hollinger metal edge boxes. For big placements I have used Solander Museum Cases...pricey.