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