Since mining has its own level-up system that allows for more efficient work, why can't letter delivery have the same? After delivering so many letters, maybe players can rank up and get slight pay raises per letter. They could even get tougher tasks like "Mail 3 letters to the spots on your map and return to the post office for your pay." There could even be time limits or special rules like "don't die" to give added challenges and a "high risk, high reward" system to reward those that complete these tasks. Jobs like these could give more variety to the letter delivery job and offer incentive to do more letter delivery while picking trash and mushrooms.