Very cursory, untrained look at the revenue/profit figures posted gave me a guesstimate of fedexprofit being 7.5% of revenue across all divisions. IF expenses didn't drop with a 2% drop in revenue, it would ALL come out of profit. But of course, expenses would drop with the loss of Amazon. BUT so would overall profit. Fedex might retain more profitable customers, and have a higher RATE of return, but overall, profit would be down, unless FEDEX is taking a loss on Amazon, which is doubtful. So the net result will still be a drop in share value.
It looks like roughly if fedex loses 2% of revenue, it will lose at least 0.14% of profit and if you project that Amazon delivers all their own packages in the future, fedex could lose ~0.5% of profit. This is all VERY rough, varying depending on the differing rates of return on ground vs express. If the majority of Amazon is delivered by the division with the highest rate of return, then the loss could be quite a bit higher.