It looks like the cart is built to be extended on the payment processing side of things ... is there a way to build shipping calculation modules (e.g., for UPS, USPS, FedEx, etc.) in the same manner?
Not really. There's no clean way to provide a dynamic shipping price in SimpleCart currently. It's possible to have a workaround where you use xPDO to create new shipping methods with the right price and apply that to the order, but that's not great.
Commerce makes it a bit easier, with custom shipping methods baked in.
Ok, thanks for your response. My challenge is that I'm in search of a native modx cart for a job in the very near future. Commerce looks promising but it appears that a production version won't be available for quite a while.