For what it's worth, I did not select 'preferred' seats. I flew economy and simply wanted to ensure that my friend and I sat together. If you already have a charge from airberlin for the selected seats, I wouldn't worry about it. I'm assuming you requested specific seats together.

I don't think I ever received a correspondence from airberlin confirming our seats on an AA award ticket that I paid for after-the-fact. Nonetheless, if you paid for assigned seats, I'd trust their system enough to not lose sleep over a trip.

As an aside, I held no status with any Oneworld airline when I purchased the seats. Since you're paying for the seat assignment, I don't think airline status means anything.