Here is the full list of all 23 Mail Time achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
you did it!! kinda... not really.... BUT you did do other things!
deliver meals instead of mail!
complete 50 deliveries!
find 40 trinkets!
sample all the rare mushrooms!
do your part & clean up the forest!
glide for 15 minutes!
deliver an anonymous love letter <3
deliver a fan's letter to their idol!
prevent a crime from taking place!!
meet everyone in the neighbourhood!
find 20 trinkets!
complete 25 deliveries!
glide for 5 minutes!
deliver a letter in under 30 seconds!
find all rare coins!
complete 10 deliveries!
take over 15 minutes to deliver a letter!
file an official letter of complaint >:(
glide for 60 seconds!
find 5 trinkets!
get to know your neighbourhood
Deliver your very first letter!