Product: Debt collection

On XX/XX/2016 I paid for parking in the City of XXXX using the XXXX XXXX app. My car license plate is " XXXX ''. I did not notice autocorrect changed the spelling from " XXXX '' to " XXXX ''. 
A parking ticket was issued as a result of the error. I immediately wrote a letter to the City of XXXX Parking Division to explain the error. The XXXX app shows proof of payment. I provided a copy of my car registration to show my license plate. I included a letter of explanation. I believed the error had been resolved until I received a letter from Professional Account Management, LLC ( P.A.M ). 
I phoned Professional Account Management to explain the error. Professional Account Management indicated I needed to appeal the Debt Notice and provide documentation I sent to the City of XXXX. I mailed the documentation that day. 
Approximately 30-45 days after I mailed the appeal to Professional Account Management I received a second Debt Notice to Collect. 
I phoned Professional Account Management explaining that I had sent the requested information to them via postal mail. I copied the documentation and mailed it again. 
Approximately 30-45 days later I received a third Debt Collection Notice. In a telephone call that day Professional Account Management told me they would elevate my appeal to a supervisor and the claim would be resolved. 
On Friday, XX/XX/XXXX I received another Notice to Collect Debt. I phoned Professional Account Management and was told there was no notation or documentation in my file. 
I subsequently attempted to send the support documents via secured signature mail and drove to the location listed but could not contact Professional Account Management directly, in person, in any way. 
I phoned Supervisor XXXX at Professional Account Management and left a voicemail requesting a return call for assistance. My message was never returned. 
In replying for the 4th time to Professional Account Management, I wanted confirmeation of my postal mail delivery to them by sending my response certified registered mail. Considering that their only mailing address is a XXXX address, I soon discovered that the USPS will not send a signature required letter to a XXXX . 
Determined to get a confirmation of receipt of my letter and corresponding documentation, I found a physical address for Professional Account Management located on XXXX in XXXX. Immediately, I hopped into my car and drove to the location to ensure that I would be able to hand deliver my documents to them only to discover that Professional Account Management does not occupy the physical address I had found on-line. 
On Monday, XX/XX/XXXX, I phoned Professional Account Management three times. On my third attempt I was told they can not reveal their physical address. Additional the person that I spoke to explained that I had to get an available supervisor 's FAX number so that I could fax my documents to them but that there were no available supervisors to speak to and was told to call back again in about 30 minutes to see if a Supervisor would be available. 
Despite repeated attempts to correct the City of XXXX initial error it remains. This matter needs to be resolved today. They are using tactics to collect a debt that is not owed but hoping that their tactics will wear my down just to pay it to make it go away.