If your users get permissions errors then you need to ensure their profile has the correct rights to the objects and field level security.
- Navigate to Setup > Manage Users > Profiles if you need to make a change.
- Ensure that these objects all have Read, Create, Edit, Delete access.
- Ensure that Visualforce Page Access is enabled for all pages that start with zkfedex.
- Ensure all the field set permission checkboxes are set. You can do this by clicking on the FedEx related [View] links, that then take you to the edit screen.