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