How do you get tickets through the mail box( and how do you send them)?

You can send thank you notes (letters) to the people who help your farm when filling crates on your boat, or by watering your withering Trees and Bushes. You just need to tap on their avatar to revive the dead plant they watered, and then tap on their avatar again to send the thank you note.

The avatar will have a little envelope showing so that you know you're sending a thank you note to that person. A message will also appear briefly on your screen, and it reads something like "you have sent a thank you note to *****". You only get a little time to do this as the avatars disappear pretty quickly after you have tapped them the first time.

When your boat crates get filled, you only have to click on the crate/avatar once I think..

to receive them, other people have to do the above for you when you help their farms.

Hope this helps :-)