I think this is an interesting idea for a game mechanic. I'm curious how it would integrate with the rest of the game (i.e. could you just always try and set up a courier quest when travelling to gain extra XP or rewards from it, or would they pop up randomly and you would have to choose whether to accept or not?)
It seems like you've thought the idea through a fair bit. If I come up with any other ideas re: mechanics I'll add them here!