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