You know the (German) saying about diging holes for other people Neji, donīt you... 
Actually I want to say that one reward per letter is a bit too much, so what about reward levels...
1 letter ->one reward
5 letters next (random) reward
10 -same as above-
15 -same as above-
30 -same as above-
50 -same as above-
75 -same as above-
...
and maybe from 990 - 999 you get everytime a reward for every letter you find
You have mistaken something. You wouldn't be collecting the 999 letters alone. You will be racing against other players to see who can find the most letters before the grand total from the whole server reaches 999.
Example: Neji => 100 letters, Baluski => 120, Xaos => 200, Lebis => 150, Kamui => 320, Gyururu => 109. Grand total => 999 (all letters have been found)
Kamui found 320/999, more than anybody else. So he wins the special prize. Something like that. The event will continue without time limit until all 999 letters are found.
To make it more interesting, you will not be able to find out who has the most letters at the moment until the result is out. You will only be able to check how many letters still remain scattered out there by visiting Sakura in the forest.
