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.
you did it!! kinda... not really.... BUT you did do other things!
deliver meals instead of mail!
glide for 15 minutes!
sample all the rare mushrooms!
find 40 trinkets!
complete 50 deliveries!
prevent a crime from taking place!!
deliver an anonymous love letter <3
meet everyone in the neighbourhood!
deliver a fan's letter to their idol!
glide for 5 minutes!
complete 25 deliveries!
do your part & clean up the forest!
file an official letter of complaint >:(
find all rare coins!
take over 15 minutes to deliver a letter!
complete 10 deliveries!
deliver a letter in under 30 seconds!
find 20 trinkets!
glide for 60 seconds!
get to know your neighbourhood
find 5 trinkets!
Deliver your very first letter!