There is a way you can customize the message. To do this, go to your Themes settings and edit languages. The default message is found in the Checkout & system tab. We have a guide on our app’s help center that can walk you through this process. Since it only refers to Shopify settings, you don’t need to use our app to follow the guide.
Please note: this is the general default message that appears whenever a shipping method is not available at checkout. So it triggers under any circumstances that result in a shipping method not being available at checkout.