These postcards are works of art

https://www.psacard.com/articles/articleview/9274/psa-dna-set-registry-classic-cooperstown-collecting-signed-1980-2001-perez-steele-hall-fame-postcards

These are standard postcard size cards and we have protective sleeves for them in polyethylene, polypropylene, and polyester (mylar). Since they are potentially very collectible – polyester might be your best bet for a sleeve. Polyester is approved by The Library of Congress as the safest material for paper products and long-term storage. Find the link here. https://www.bagsunlimited.com/category/96/sleeves