I have bought a few things used and ended up with the original receipts for one reason or another. It's a little snippet in time and reminds us all how much the monetary value of our LD hobby has dropped. Every time I pick up a box set I'm glad I didn't pay the original price.

Panasonic LX-900U with receipt and warranty. Originally paid $600 in 1995

The Abyss box set from Suncoast. Originally paid $99.98 in 1993

The Crow from Suncoast. Originally 39.99 with a 5.00 markdown and used a gift certificate in 1994 (back when MN sales tax was only 6.5% Those were the days)

I've ended up with all sorts of odds and ends tucked inside some used laserdisc sleeves.
Quite often music titles come with stickers, magazine cuttings of the artist and other memorabilia as well as the receipts you mention.
I've even encountered quite a few cases of two sets of the Japanese paper inserts inside the same sleeve....not sure what was going on there!