A lot of research needs to go into making a decision on a year-long contract of any kind, especially with suppliers. I think suppliers expect to negotiate with their customers just like unions negotiate with management. Negotiating price with suppliers is just as serious as negotiating wages, and should be treated in that manner. Ask for a second meeting. Consider not just this one issue on your performance rating.
Performance is clearly an emotional issue.
No employee wants his or her work ethic questioned. Getting a second opinion might help to bring some unemotional perspective to the issue. If none of this works, consider how important this one performance rating is to your career. If it is critical and you are a member of the union, ask for union representation when you meet the second time with your boss. Remember, your boss was embarrassed.