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