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