Speak to Trelawny at camp following the completion of Magician’s for Sport and agree to help him with a job he’s got lined up. There’s no shooting in this mission and it’s a purely time based Gold Medal so consider going for the Gold Medal on a replay, that way you can safely skip all the cutscenes to save time where possible. Follow Trelawny to the Rhodes Post Office and hitch your horse on the hitching post outside. Upon heading inside, pay Alden and he will hand over knowledge of a stagecoach coming through. Head back outside and mount the horse, following Trelawny to the next marker.
As Trelawny trots off to begin his performance, hang back with Arthur so you’re not spotted. You can move slowly up to the road sign and you’ll see a bush off to the side of it. Crouch down and wait for Trelawny to get Mrs. Damsen to exit, the nearby guard will then head down to the river bank giving you time to crouch walk as fast as possible to the back of the wagon and break the lock. Take the money and then retreat the way you came as quickly as possible, once the stagecoach leaves, head back to Trelawny to complete the mission.