Here is the full list of all 20 KeyWe achievements.
The base game contains 16 achievements, and there is 1 DLC pack containing 4 achievements.
Complete every level in Winter
Earn Postmaster on all Winter levels
Earn Postmaster on every level in the Calendar
Complete every level in Autumn
Earn Postmaster on all Autumn levels
Earn Postmaster on all Summer Levels
Complete every level in Summer
Find a Collectible
Complete the first week of Summer
Beat your previous score on a level
Become registered Teleposters.
Complete an Overtime Shift for Bonus Pay
Earn Postmaster on every Overtime Shift
Earn half of the wearables in the Wardrobe
Earn all of the wearables in the Wardrobe
Find every Collectible