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!
glide for 15 minutes!
find 40 trinkets!
complete 50 deliveries!
deliver an anonymous love letter <3
prevent a crime from taking place!!
meet everyone in the neighbourhood!
sample all the rare mushrooms!
glide for 5 minutes!
deliver a fan's letter to their idol!
complete 25 deliveries!
do your part & clean up the forest!
find 20 trinkets!
glide for 60 seconds!
deliver a letter in under 30 seconds!
complete 10 deliveries!
take over 15 minutes to deliver a letter!
file an official letter of complaint >:(
find all rare coins!
get to know your neighbourhood
find 5 trinkets!
Deliver your very first letter!