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