I've created achievement file:
http://pastie.org/9478412
My CB config:
http://pastie.org/9478417

Now, if player has already 15 posts written and logs into the game for the first time, he'll get all rewards for all tiers up to 15 posts.

But if he already got the reward for 2 posts, then stay in game and write on forum another 13 posts, he'll get nothing until he relogs to the server, and even then he'll only get cash (no items).

If the player is not online while reaching 15 posts, and then logs in, he's getting all the rewards as he should.

Rewards are only performed during synchronization. So if they're playing and qualify for a new award they'll receive on the next scheduled synchronization of all players. Or otherwise they'll receive it upon logging out.

Items sounds like a separate issue...I'll have to run some tests to see what is going on there.

Something else to watch for is that if the player has insufficient room in their inventory to receive ALL the rewards, they won't receive any part, not even the cash, until there's a synchronization event when they have enough room in their inventory for everything.

I did my tests on the test server with absolutely empty inventory to avoid any mistakes. I thought that it might be relying on synchronization, but still money was given after relog, and items were not. That's what made me thinking that if you are online while writing posts, material rewards won't be given at all. And to make it clear, if I was offline while writing posts, on login all rewards both material and cash were given.