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