Regulations and Procedures: Paid Auto Registration by Mistake, County Won't Refund or Credit Account

We traded in a Ford car registered in Colorado to a car dealership and purchased a Ford truck at the same time in November 2015. For some reason, we have received a bill for registration from the county for the car for the past two years even though we don't own it. My mistake -- I paid the registration fee each time without noticing it was for the car we no longer owned. We were finally informed by the county this year that state records showed we no longer owned the car. Fortunately, we were able to talk the county into refunding the 2017 registration fee; however, they refuse to refund the 2016 fee we paid in error. Do we have any recourse?

Regulations and Procedures: Paid Auto Registration by Mistake, County Won't Refund or Credit Account