copied this from a web site...... "The United States Express Company, which operated from 1854 to 1914, was a privately owned company that forwarded parcels and freight. The company served the northern states from New England west to Colorado."
I believe that the US Express Company was absorbed by Wells Fargo, although I'm not 100% sure. The stamp could be worth some decent money. I suggest you devote some time to do some research on it.