The api call
GET https://api.softlayer.com/rest/v3/SoftLayer_Product_Package/257/getConfiguration?objectMask=mask[itemCategory]
returns an
The default endpoint is REST but in your environment you are using xmlrpc, which is probably due to the existence of the ~ / .softlayer file and in this it is configured as an XMLRPC enpoint.
For more information you can see the following documentation: https://github.com/softlayer/softlayer-go
This issue happens for the XMLRPC enpoint and you can report it in the go GitHub. https://github.com/softlayer/softlayer-go/issues/
Try in your code with the REST endpoint, like this example:
endpoint := "https://api.softlayer.com/rest/v3"
// Create a session
sess := session.New(username, apikey, endpoint)