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