BillMatrix review: Invalid checking account

Bill matrix made an error, but would not do the research to see where the error occurred. I made a car payment on 2-11-13 via online to bill matrix. On 2-19-13, I received a letter from Nissan motor company informing me that my payment was not accepted by my bank due to an invalid account number. Even though I always check both the routing and checking number no less than 3 times on any transaction of this type, I realized that I could have incorrectly put in my account number. Made a phone call on the same day as letter received and tried to make the payment over the phone with no success. Was told that the routing and checking number were invalid according to my bank. Was told to go my bank and have them type a letter on their letterhead verifying that my routing and checking were open and valid. This was odd, bill matrix could see 4 years of payments through them with the same banking information. So, the next day - 2-20-13- I made a trip to my bank and the bank was helpful enough that they voluntarily called bill matrix to talk with a rep. The bank told bill matrix that they checked the history and they never showed this request for payment to have ever come through to them. The bank also verified that the banking information was correct, but bill matrix insisted that they still had incorrect info and needed the letterhead. The bank graciously typed a letter and faxed it to bill matrix, but the matter wouldn't be resolved for 7 to 10 business days... which of course would make the payment late, causing a late fee and a negative impact to the credit bureaus. But wait, to avoid this, bill matrix bill would take a payment from my check card right then(by the way this is the same check card tied to the invalid checking account?). The check card payment went through right away, the bank rep and I could both see the transaction immediately. BUT, before they would accept this form of payment, I had to agree to a $12.95 convenience fee. Whose convenience, certainly not mine, the customer. The error was apparently not mine or my bank, but I had to spend by time in addition to a convenience fee, for something bill matrix had no desire to research even after they verified with me and the bank that the banking information given was correct. Again, the request for payment was never received by the bank...but none of the reps(3 different people during this ordeal were involved) seem curious as to how it could happen. I have no recourse but to continue to make my car payment through bill matrix since this is the car dealerships choice of payment, but I can at least voice my frustration with both. Utter frustration and still no explanation as to what happened.