This error you are getting makes sense for the results you are seeing. The problem is like the error says, the value you are referencing has not been defined. Since this is on the post value it tells me that you do not have a form element on your checkout form with this name that is posting to your confirm page.
To fix this you must ensure that the input on your form that you have the user select the county from is name countyid. If you have trouble getting this in place please post back with your checkout page.