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