First off, call the Cust. No-Service number and ask to speak to a supervisor. But, don't threaten to cancel the card. If you cancel the card, you lose all bargaining power. Ask them why your promotional rate was removed, and indicate that they need to restore it if you have not violated any of the terms of your agreement. Once the card is paid off, then cancel it. Now, one thing you may not realize if you didn't read the fine print -- Chase requires your payment to be in their hands PRIOR TO 8:00 a.m. on the day the payment is due. If it reaches their lockbox, or their internet payment system, after 8:00 a.m., it is considered late.
Second, don't just vent here (nothing wrong with that, just don't stop here). You need to contact Disney and let them know of your displeasure with their business partner. Your letter may not change anything by itself, but if they never hear, they won't know that a problem exists.
Third, and this is personal opinion, I will NOT do business with Chase Bank for anything because of their poor customer service and issues I have dealt with in the past. I recently told a car dealer I would not buy a vehicle from him if he only had Chase Bank financing. They, and Bank One that they purchased, have been the subject of multiple class action suits.