Withhold payment first (Score 1)

The obvious problem is that you made final payment before testing the script. With that payment you gave away all your leverage. The time to worry about getting ripped off is now. The sqeaky wheel gets the grease, so squeak loud and repeatedly. Do it in public if they ignore you for long. In the future, don't begin a project without a clear and detailed scope of work in writing that details payment terms. Ensure that the majority of the money is payable only when the scope of work is complete and code has been tested to your satisfaction. If a vendor won't agree to those terms don't do business with that vendor.