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