Is there a charge for nitrogen if I purchased my vehicle from Sir Walter

No

Mr. Kelly - no charge! Bring it in anytime and we will get you squared away. Just tell them Mr. Birckhead sent you in to get topped off / refilled with no charge. - Matt

Thanks, King Chilly. I didn't think to ask that when I bought my Sonic. Good save.

I don't think so. But you can always ask them