The Diplomatic Service is the corps where all the King's Couriers are in. They bring messages all around Araluen in order of the King, and occasionally get very dangerous missions with the possible risk of being captured or killed. It is known that the King does not pay ransoms for couriers, but will take revenge when he arrests the one who captured or hurt his Couriers.
The only members that we see in the books are Alyss Mainwaring and Lady Pauline. Pauline is Halt's wife and is in charge of the Diplomatic Corps of Redmont Fief. Alyss, Will's wife, is serving in that Corps as well.