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