I've just run some numbers: In order to recoup, in fuel savings, the additionaloutlay for the Cayenne Diesel you would have to own it for about 11.9 years.
While there will be up front costs of purchasing the new vehicles, over time the enormous fuel savings will more than make up for this initial outlay of capital, and Fedex is also gaining a reputation for being socially responsible, thus potentially attracting additional customers who would have otherwise used a competitor.