Here is my achievement guide, for every single achievement in Turnip Boy Commits Tax Evasion.
All of the achievements are ordered and have chapters attached in the video.
For this achievement, you will need to beat most of the game, and get into the bunker below your home.
Make your way through a few rooms, constantly progressing west (left). Once you maneuver a watermelon through portals, and there's a western room to enter, the letter will be in here.