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