I went back and re-read the ad...kinda makes me think it's $300 plus the core...but, I sent him a question about it...
Will post what his response is...
Got a couple of responses back from him today...
Pay him $300...send him your seat...pay him $25 for return shipping...your custom seat will show up pretty soon...
or
Pay him $300 plus $100 core charge plus $25 for shipping...he will ship you a custom seat...you send him your good core seat...he will refund your $100 core charge.
Seems pretty straightforward...but, yeah, I also wondered about how that cover was fitting in the picture...
Hmmm...?