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.
take over 15 minutes to deliver a letter!
complete 10 deliveries!
deliver a letter in under 30 seconds!
file an official letter of complaint >:(
glide for 60 seconds!
find 5 trinkets!
get to know your neighbourhood
Deliver your very first letter!
you did it!! kinda... not really.... BUT you did do other things!
meet everyone in the neighbourhood!
complete 25 deliveries!
complete 50 deliveries!
glide for 5 minutes!
glide for 15 minutes!
find 20 trinkets!
find 40 trinkets!
deliver meals instead of mail!
sample all the rare mushrooms!
deliver a fan's letter to their idol!
prevent a crime from taking place!!
deliver an anonymous love letter <3
find all rare coins!
do your part & clean up the forest!