I collect American pre-1920 book trade covers and postal cards--- publishers, booksellers, bookbinders, etc. It's a supplemental collection to my book trade label collection. I try to avoid getting material for magazines/ periodicals, but quite a few have snuck in over time.
Here's my oldest postal card in this collection, June 1873 from bookseller Estes & Lauriat to bookbinding company George H Tilden:

