Hermes Express

The Hermes Express is the Olympian's equivalent of a courier/messenger service.

The Lightning Thief
Percy uses the Hermes Overnight Express to send the head of Medusa back to Mount Olympus after he beheads her in the battle at Aunty Em's Gnome Emporium. Later, he finds the same package in his bedroom marked "Return to Sender" sent back by Poseidon.

The Sea of Monsters
Hermes mentions to Percy that the Overnight Express business is booming and later Poseidonalso uses the service to send Percy a letter of which Hermes delivered himself.

Percy Jackson and the Staff of Hermes
While Percy was celebrating his one month anniversary with Annabeth, a Hermes Express truck appeared on the lawn in front of them. No one else in the area noticed it except them and Hermes came out of the truck, asking Percy a favor to find his Caduceus so he could make his deliveries on time. Percy returned the staff and Hermes finished his deliveries.