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