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