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!
deliver a fan's letter to their idol!
deliver an anonymous love letter <3
complete 50 deliveries!
sample all the rare mushrooms!
find 40 trinkets!
find all rare coins!
prevent a crime from taking place!!
complete 25 deliveries!
meet everyone in the neighbourhood!
take over 15 minutes to deliver a letter!
glide for 5 minutes!
find 20 trinkets!
do your part & clean up the forest!
complete 10 deliveries!
file an official letter of complaint >:(
glide for 60 seconds!
deliver a letter in under 30 seconds!
get to know your neighbourhood
find 5 trinkets!
Deliver your very first letter!