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