Only allow FedEx shipping for certain product?

You would have to use a shipping class for this and make this class have a higher priority..

Here we are: https://wcdocs.woothemes.com/user-guide/product-shipping-classes/

“Certain shipping methods, such as Flat Rate Shipping and Table Rate Shipping, can make use of Shipping Classes”

But can FedEx and USPS extensions make use of shipping classes too?

To be more specific:

Customers should be able to select a USPS or FedEx realtime rate method when they’re ordering books or DVDs. For the bigger, fragile item, the USPS options should not even appear, only the FedEx.

You can do this with this extension.

https://www.extensionworks.com/product/woocommerce-usps-shipping-extension/

https://help.extensionworks.com/entries/22826147-Installation

You can choose a shipping class for your product and in there extension you tick only ship items with the USPS shipping class.

The same for there FedExShipping extension.

@dax702 – As mentioned by @angelme, the Extension Works plugins have a shipping framework that allows the extensions to be more granular – i.e. specify custom boxes.

You can also specify at the product level the shipping method/service that will be used.