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.
deliver meals instead of mail!
you did it!! kinda... not really.... BUT you did do other things!
glide for 15 minutes!
find 40 trinkets!
complete 50 deliveries!
prevent a crime from taking place!!
do your part & clean up the forest!
sample all the rare mushrooms!
deliver a fan's letter to their idol!
deliver an anonymous love letter <3
complete 25 deliveries!
deliver a letter in under 30 seconds!
find all rare coins!
meet everyone in the neighbourhood!
find 20 trinkets!
glide for 5 minutes!
complete 10 deliveries!
file an official letter of complaint >:(
take over 15 minutes to deliver a letter!
find 5 trinkets!
get to know your neighbourhood
glide for 60 seconds!
Deliver your very first letter!