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