1st: go to 1/3 point, leave 1/3 load of supply, go back;
2nd: pick up the 1/3 load that was left from the first trip, (fully loaded again now.) make another 1/6 trip to the half point. A the moment he has 5/6 load, leave 1/3 load at half way point, go back with the rest 1/2 load.
Repeat the above 2 trips twice. Totally make 6 trips. Then there is a full load left at the half way point.
7th: pick up 1/2 load at the half way point and become full-loaded again, continue to the destination and pick up the letter, come back to the half way point. Take the rest 1/2 load and go back.