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