ITP VoIP ITP is withholding credit for late return of modem, Internet

I was charged $99.99 for not returning the "free" phone modem.  I called ITP to see if they would credit the $99.99 if I returned the modem.  They said absolutely they would.  So I returned the modem via USPS mail with delivery confirmation.  ITP received the modem on August 4, 2010.  I called to make sure they received it, and the customer service rep confirmed receipt.  She also said the credit would be processed in 3-5 days.  It is now two weeks later and the credit has not been issued.  I now have to dispute the charge with American Express.  What a pain!