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