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