Great advertisng cover from Levis Strauss & Co., San Francisco with a double postmark. San Francisco on the front and San Andreas on the back with the date October 20, 1887. It looks like a stamp collector clipped the stamp from the cover.
The envelope looks to be addressed to C. Agostini in San Andreas, California. I did find a Cyprien Agostini while digging around online. If it is the same person, he was the owner of the D'Agostini Building which was built in 1851. Agostini purchased the building in 1870. It was the first brick building built in San Andreas and one of only a few to survive the disastrous fire of 1858. After attempts to save the building as a landmark by the Calaveras County Historical Society, it was dismantled in 1971.