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