I've had the opportunity to handle quite a few originals; this is certainly of the type. Soldier's used all sorts of paper as well. I handled a letter written on the paper from an St Louis arsenal pack and another from the Indianapolis arsenal.
According to the NS Trader Price Guide a color patriotic unused cover, ranges from $20-$100; I'd say yours is closer to the upper end as it is early and deals with an important figure. I only use the price guide for items such as this and not cloth or weapons.
Neat envelope, I have a collection of family letters written during the war my grandparents passed on to me. This one is the most interesting being it has the envelope and was written on patriotic paper.