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