You will have to take him to court for the money he owes based on the contract. As for the design patent, you should be protecting the appearance of the assembled kit and the novel parts of the kit via design patent. Those parts that were solely created by you, you could file for in your own name and those parts and the overall appears of the assembled kit with those jointly created parts would be filed for in both of your names. If the customer refuses to sign the papers, there are procedures within the US Patent and Trademark Office to deal with such circumstances. You need to see a contract lawyer for the breach of contract issues and a patent lawyer to handle the design patent filings.
Answered on Aug 17th, 2012 at 8:51 AM