I just bought a new vehicle last month and put the whole purchase on credit card (about $26000). I offered to pay the merchant fee (they quoted 1.8% for Aerogold VISA) as the points are more valuable to me. In the end, they never charged the fee though. I got 26000 points, a great deal on the vehicle and didn't have to pay the merchant fee. Everyone seems to be happy. The dealer even sent me flowers afterwards!^
I wouldn't put it on AMEX as there would have been a possible 3% fee but only 25% extra points.