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