How does FedEx Smartpost work?

Many of our shirts are sent via FedEx Smartpost.  We give the package to FedEx and then one of two things can happen.  FedEx may take the package most of the way and drop it off at a US Postal Service facility for final delivery or FedEx may decide to deliver the package themselves and a FedEx delivery person will make the delivery.  FedEx is completely in control of this decision and the decision is made based on what will be the most efficient route based on promised service levels and cost.