Sometimes the related nonphilatelic items are just as interesting (or even more interesting) than the stamped items themselves. Exhibitors frequently seek out these items to add context and/or flavor to their exhibits. I don't exhibit, so to me they're just "cool related items".
Here are some examples that I've picked up over time.
1. This tattered illegal use cover was part of a group lot of illegal usages offered on eBay
in April of this year. All were either out-of-period usages (so likely philatelic) or had condition issues. The lot ended going inexpensively. When the lot arrived, the cover was enclosed in a letter... which had not been mentioned or pictured in the listing. It's a 1956 letter from Hugh Shellabear, revenue collector and author, providing context to the letter's attributes. The selling dealer was a revenue specialist, so they should have known the significance of the author. Why wouldn't they have shown the letter? It's a great selling point!
2. The fish imprint RN checks from John Elsey & Co. have been mentioned here on the forum before. They are great (and scarce) visual and topical items. I love the fish-shaped check protector imprint. Just scouring eBay
for billheads I stumbled across the companion billhead, outside the taxation period, showing a similar vignette. A bit worse for wear, but makes a great companion piece.
3. When I purchased my first example of R106a (4-cent 2nd issue bisect), it included the original 1981 letter from Sherwood Springer confirming it as the Scott Catalogue listing example.
4. When I first encountered a document from the Market Fire Insurance Company, I was immediately taken by the ornate red and black engraving. I came to discover that other documents from the company were similarly ornately engraved, so I sought them out.
5. Lastly, probably my alltime favorite pair of related items, a horse theft insurance policy and companion advertising envelope from the "Pennsylvania Mutual Horse Thief Detecting and Insurance Company". The piece de resistance is the embossed company seal depicting a horse thief being chased.