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