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