I don't think it's a native feature yet as we'd love to have SurePost as an option too and can't seem to find it. I'm pretty sure (90%) that you have to use a 3rd party app like PlugInHive's Multi Carrier Shipping Label app.
Hope that helps, but I'm going to monitor this thread as well in case there is a better solution now.
As Jeff has mentioned, you can use the Multi Carrier Shipping Label app to handle UPS SurePost as Shopify doesn't directly support SurePost. The app allows you to configure your own UPS account with negotiated rates and easily enable SurePost. (see the image below)