FedEx: Add option specifying which rate type to return (Retail, Negotiated, etc.)

[dmmikescott]

It looks like the FedEx provider is returning the highest rate returned for each shipping option, so there is no way to get the negotiated rate for the account submitted. For instance, I'm testing shipping from Wisconsin to Canada. The rate on two of the RatedShipmentDetails are $23.90 (negotiated rate), but on the other two they are $70.65. Huge difference.

[carlosredondo]

It will be nice to have a flag (on/off) for negotiated rates. Some customers wants to make negotiated rates available to final consumers while others don't.