Our will return shipping rates from FedEx API. If you have the shipping rates negotiated for your FedEx Account, then once you configure the plugin with your FedEx account details, the same rates will be displayed from plugin as well as they are returned from FedEx API.
Also in order to confirm that FedEx shipping services for the Mexican addresses, you can check with the FedEx Price calculator, here is the link:
Hope that helps!
Let us know if you have any query.