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