Emily T Thayer (b.1848, d.1896), daughter of Collins Thayer, never married, buried Elmwood Cemetery, Norfolk,VA. Her sister Susan A. Thayer married Charles Guild and moved to Baltimore where she died in 1899.
Your stamp is a Scott #65, issued in the fall of 1861 as the workhorse letter-rate stamp of the 1860s. Absolutely huge quantities were printed. Your envelope appears from 1863 based on the faint year date written sometime later to the left of the cancel. This would also be consistent with the blue cancel which Baltimore was using then, rather than 1867 as the title line suggests.
I agree with oldboldandbrash, the monetary value is quite modest.