(Not $rand isn't in yet)
Code:
%r = "$rand(0,10)";
EVENT_ITEM {
if( $itemcount(1001) == 1 && %r >= "5") { say("Thanks heres your reward.") summonitem("1010") }
if( $itemcount(1001) == 1 && %r >= "8" && %r < "5" ) { say("Thanks heres your reward.") summonitem("1011") }
if( $itemcount(1001) == 1 && %r <= "9" ) { say("Thanks heres your reward.") summonitem("1012") }
}