American Express Stock Rose 30% in 90 Days. Here's Why It's Still Undervalued.

American Express (AXP 1.47%) stock has been a rocket. During the past 90 days, the shares have risen by more than 30%.With the stock trading at all-time highs, it's reasonable to question whether the valuation has gotten out of hand. The stock is no longer the bargain it was a couple of months ago, but there are many reasons American Express remains a wonderful buy-and-hold investment.These numbers tell a promising storyIf a certain investment rises considerably in value, that doesn't always mean it's becom ...