For FedEx there is a big price difference between First Overnight and Priority or Standard Overnight. The price for First is nearly triple the cost of pri and std. I think every seller probably uses pri or std. If you call fedex and tell them you have a business and currently ship X amount weekly with ups but you want a discount they will almost always set you up nicely just based off what you tell them. Now if you fail to ship or hardly ship they will take that discount away but it will take alot of time before they contact you to discuss. I have an account with approx ~45% off published rates and I ship sporadically world wide with my other business and have never had an issue with them revoking my discount.
Also what helps is to find out who your rep is for your region. Make friends with him, take him out to lunch have a beer. Hes got your back with a lot of pull and will take care of you. Don't under estimate this piece of advice. Good luck