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