Authorize.net users connecting via WebLink or ADC Relay Repsonse need to implement a Referrer URL on their account.
Shopsite does not use these methods to connect to the Authorize.net system. Shopsite uses "ADC Direct Response", and does not require this Referrer URL be enabled. In fact, turning this feature on may cause problems with your Shopsite store.
Users of Shopsite and Authorize.net should not use the Referrer URL setting in their Authorize.net account.