The instructions for disputing a billing error are printed on each paper/PDF statement. Do not call. Do not email. Send explanation through the USPS to the address provided. Do not include any opinions, do not make any threats. Just state the amounts paid, the amounts refunded, and specify what you want them to do.
$300 may be the minimum required payment. There isn't nearly enough detail to know.
I would also send any correspondence via USPS certified mail, return receipt requested with the tracking number listed on the letter - in my experience, when a letter is sent with tracking and proof of receipt, there is less chance of it "falling through the cracks"