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