One thing to remember is that many small tradesman may pad their bill if they know you'll be using a credit card. And if you pay their cost (Probably 3-4%), you're losing big time. Miles are nice, but not if you pay 4 cents per mile.

One thing you could do is see if any of their suppliers take credit cards (many do), and negotiate you paying for the parts direct. Now most tradesman will mark your parts up, so you may have to give them a certain percentage markup.