For each shipping service you add, you also have the option of creating shipping labels. This enables you to link a shipping service to a group of offers.
From the "Shipping services" page, click "Add shipping service"
Select "Supplier", "Type of service" and create a label (e.g. "Oversized")
You can then use your product feed to associate offers with shipping labels (
shipping_label), which you can read more about here
This way, you can control which shipping services are available for which offers.