To many (and in particular LARGE) shippers, a "rush" delivery means that they change the order in which merchandise is picked for shipping in their warehouse /distribution center, rather then changing the service level they pay the carrier for.
I am aware of one large e-tailer that can ship UPS or FedEx ground from their several distribution centers in the US and hit more then 90 % of the US population. When they charge a customer for upgraded "shipping", the extra money all goes in their pockets, and the goods are shipped ground, two days earlier then for normal shipping.