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