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