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