Here's what I've learned, BTW, I'm a FedEx guy.
When I go to an Office and show my ID, the cost is always less than when I ship the same sized item and charge it to my account. I truly think that the folks in the Office locations are applying the wrong discount code. When going through your formal online account, the correct discount is applied.
When I flew for another airline, I had to drop a heavy, specialized battery off for shipping. Since it was hazmat, I needed to take it to an actual FedEx hub. I showed my ID, told the guy what it was, and he charged me $5. I told him he had to be wrong, but he said "Nope, all airline employees get this price."
I always try and ship and pay up front.