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