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