I constantly mail my villagers stuff they put up in retail with the message, "Well, here you go." (because there's never an inappropriate time for Spongebob references)
I wrote letters to my villagers nonstop even after I discovered you could obtain badges from it I received my gold letter badge rather quickly because of my nonstop writing xD I don't write much anymore though
I got the gold badge for the letters and then stopped writing them to my villagers rofl. I occasionally do it to send them furniture or to wish them a happy birthday on their birthday. They really like the letters they get on their birthday!
I stopped writing frequent letters to my villagers. Now I usually only do it during birthdays and special occasions or when they first move in. Its too annoying to type on the 3ds, and to send it to 10 villagers, nope. Other times I'll send everyone the same shirt in failed attempts to get us all matching, or when I have a lot of reorderables as gifts from the mail that I'm too lazy to sell, I'll send it if it seems like it'll match them.
Not really- I usually just do it when I want them to put furniture in their house or change their clothes. It took me over a year to get the bronze letter badge Most of my letters say stuff along the lines of "Hi! Thought you might like this for your house. Have a great day!" although one time I told O'Hare that he and Carmen should get married XD