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