Drupal Commerce: How to prevent (or suspend) checkout where no shipping rate is defined? I am using the physical products module to calculate weight based shipping rates.

My client sells heavy items for which they sometimes need to arrange negotiated shipping rates for offshore delivery areas, such as UK Highlands and Islands. 

Because a shipping rate cannot always be provided, this can result in a situation where the system returns a shipping service option of  'No shipping rates found for your order. Please continue the checkout process'. 

From this situation it is then possible for a buyer to complete checkout without incurring a shipping cost at all!

What is the best method for preventing (or suspending) checkout in such situations? In such a way that the shop owner can provide a quotation based shipping rate before the buyer resumes checkout?

I understand that the shop owner can manually edit a cart order to include a manual shipping rate and I am also aware the Rules can be used to direct the order flow. 

But how to prevent (or suspend) checkout before a shipping rate has been quoted?
Look at https://www.drupal.org/project/commerce_rules_extra (Commerce Rules Extra) which allows you to alter (including disabling) checkout panes based on conditions.