The why is the tighter constraints of higher php versions and that most previous software was not written to avoid notices (which are to eventually become errors as reported by php).

Would suggest either providing a copy of the code in this area and/or comparing to almost any of the other shipping modules provided with the ZC core shipping modules to see how such not yet defined variables are handled.

It may also be that the module needs to be reinstalled (from within admin->modules->shipping) to ensure that the database has the latest configuration values incorporated...

May also want to identify which fedex module (ie. Link to the plugin) is being used and verify that it is up-to-date with the latest.