FedEx Collect (Authorized Accounts Only) Option
Both of my big clients use FEDEX Collect (Authorized Accounts Only) option. This lets them pay for the shipping without giving everyone their account number.
See Snapshot of how FEDEX.com does it:
Brian Schmittdiel HIPERFIRE commented
One of my big clients has recently switched to FedEx Ground Collect and requires that I use this method for shipping. Please add this to the FedEx integration.