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