Ca. made me pay tax there when doing the transaction...
I think you have to plan ahead, my brother bought a pickup in Oregon and I had to be at our local DMV when the papers were done. The pickup was originally registered here, paying only our registration and tax. Papers were faxed to our DMV, I wrote the check. Paperwork was faxed back for my brother and the plate was mailed to him. He was working there at the time, but had the paper plate faxed to him if he had wanted to drive it back right away.
At this point I think you will have to pay again to get the car registered. Sounds like they are treating it as another registration.