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