http://www.glassdoor.com/Salary/FedEx-Ground-Delivery-Driver-Salaries-E16846_D_KO13,28.htm
Looks like the average is 35k. Out of 14 people who reported their salaries no one even reached 50k (seems to TOP OUT at 45k), so the idea that it "starts at 50k" is completely ridiculous. I also could not imagine a Fedex Delivery driver earning more than a University professor, Corporate IT Technician or other jobs that actually DO start at 45-50k. Maybe in the heart of NYC where the cost of living is 100-150% higher than the rest of the US it might be possible.
Climb quickly? To what? 60k? 80k? LOL. You think Fedex Drivers are as hard to find and retain as Nurse Practitioners or other occupations that require education? I don't think so. There is a large supply of people who are 21-64 with clean driving records and the ability to lift 60 lbs along with a high school diploma (if thats even required), there is just no way the logistics companies would pay upper middle class salaries to simple drivers.
Based on the data I showed, I think more than likely they start you off at around 28k ($13.50 an hour) and if you're lucky to be living in a high value/expensive city and you stick around for a while plus put in some overtime you can get up to 45k which is still amazing pay for manual labor.
Anyway, I wouldn't wager one way or another if they took it or not. You would think TSA agents wouldn't risk their jobs either to steal from people but since the TSA was founded, hundreds of agents have been convicted of theft right from the Airport Security line stealing stuff from under people's noses.
It is very much possible the driver could have stolen it and then was going to claim the package was misplaced in the chaos, fell off the truck, or was never given to them. You never know. Oftentimes without direct hard proof the most the company can do is fire them which makes no difference if they were planning on quitting anyway.