American Express is finding that even for a provider of credit cards to the affluent, it pays to focus on the highest of high rollers.The company recently shifted marketing dollars to its refreshed Platinum card, which carries an $895 annual fee, and away from no-fee cash back cards, CEO Stephen Squeri told analysts Friday after the company reported fourth-quarter results.Doing that helps to boost overall spending levels as high-end consumers continue to thrive, a tactic that leads to both rising fee revenu ...