Drupal Commerce. Trigger shipping rules based on product field value
I eventually managed to get this working by dropping the idea of having custom fields and replacing them with taxonomy terms. For whatever reason (probably inexperience) I could not expose my custom fields to the rules module in order to trigger the shipping rules that I wanted.

By leveraging the Commerce Rules Extra module I was able first to trigger my shipping rules based on taxonomy terms instead of custom fields and secondly to define additional conditions for firing these rules with the help of the modules maintainer @MarcElbichon as discussed here https://drupal.org/node/2087815