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