Hello, @CraigAB .
Elias here with Shopify Support. That is a great question.
This feature is available to all merchants now, simply upgrade your shipping to the shipping profiles and add your third-party fulfillment service. Third-party fulfillment can be a custom fulfilment service or fulfillment by Amazon.
After that, you can enable multiple locations and begin assigning inventory to locations. Once that’s done, go back to the shipping profiles and set up your shipping rates for each location. If you have vastly different rates between products, you can consider using the combined rates at checkout.
Is this the feature you were looking for?