What is the highest level you can get mysterious letters?

I had to wait until Level 60 something before I got my fourth letter. The island is like a chest - it gives out a certain number of items then re-charges. After the second lot of items it disappears and you have to collect letters again.

I just got one, I'm at level 46 and i just upgraded to the level 7 bucket. It's the 5th one so far. Hope this helps!

I actually threw away the first envelope I got… I know I was STUPID, I just hope I can get more and I think there is a chest also

You merge 2 letters to get an envelope, merge 2 letters to get an island. That is the highest. When you tap the island you get gems, coins and rare fossils.