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