Hello Ian,
Using a test account will be the same as using a real FedEx account, because the FedEx shipping plugin will only :
- Send details about your order details (Products weight/dimensions, shipping address, ...)
- Retrieve and display rates returned by the FedEx shipping platform
So you should probably just use your FedEx credentials. But if you really want to use it in sandbox mode, you'll probably have to directly edit the code of the FedEx shipping plugin, to change the URL used for the request.