Hello, late reply but here is the response from Shopify support. This is indeed not possible. 

Thank you @ShipVista_COM, the area is empty as we never set it up. But the option still appears on Shopify POS.

Hi all,When shipping to customer from Shopify POS, I would like to know if it is possible to disable and hide the "Custom shipping rate" option. I couldn't find anything about this.Any help would be very much appreciated! Thank you!