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