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