See first quotation.
1844. “Drop letters.”… This is a class of letters which are usually sent by private conveyance, and are “dropped” or deposited in the post-office for delivery.—‘Report of the Post-Master General of the U.S.,’ p. 688. (N.E.D.)
1888. A St. Louis exchange wants drop-letters reduced to one cent.—Chicago Inter-Ocean, n.d. (Farmer).
