a few rules that I live by (others may not agree):

1. never, ever bite the hand that feeds you.
2. you will catch more flies with honey than with vinegar.
3. fool me once, shame on you; fool me twice, shame on me

meaning: if this is your only source, and you want to keep buying, keep it civil. I would give him another chance and if it happens again, then I would say something - polite but firm. Remember, it's a business deal, and you should use the same tone of voice that you would with your boss when asking for a raise.