Looking for old business mailed envelopes or missionary correspondence - $100 (Los Angeles)

I am a collector of history and paper items.
Have you found an old personal or business correspondence? Old envelopes (with or without the letters), postcards etc? Anyone in your family a missionary from way way back and sent letters and cards from far away islands and exotic places? Do you know where some is? You could turn that lucky find into some serious cash! I buy old envelopes (with or without the original letters), and postcards that went through the mail.
Boxes of these are often found in business archives, estates or attics, or it may be a portion of a stamp collector's collection.
pre 1970 only please.
I do NOT buy first day covers, commemorative gold stamps, etc.
What looks like worthless paper to some people can become real cash in your pocket!