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