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