Here is the full list of all 19 Return to Sender achievements.
Purchase 5 peashooter upgrades as Stuart
Complete all special runs with Stuart
Complete all special runs with Morris
Complete all special runs with Gunther
Complete all special runs with Brutus
Complete all special runs with Humphrey
Complete all special runs with each character
Defeat Commander Cacti
Defeat Chum
Defeat Hothead
Defeat Croc n' Ball
Defeat the final boss
Reach the Lower World
Completely use an altar and be left with less than 5 HP
Kill 5 or more flies at once using the Fuzzy Socks
Kill a nomad by Parrying one of their parrying shots
Unlock Gunther
Unlock Humphrey
Buy 25 items from the item shop