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