Set unique identifier for shipping methods

On our website we have a number of different shipping methods in the same shipping zone. We are now building a new website that has the same products and shipping methods, and we will be migrating to the new website shortly. In our current e-mail notification functions (and other functions where shipping methods are conditional), we are using shipping instance_id to differentiate the shipping methods. Since this id is generated by WooCommerce, there is no way to match the shipping methods from the old website with the ones in the new website. We don’t want to use method_title since it’s displayed in the checkout and might be changed in the future.

To my question: does anyone know about a plugin that allows you to assign your own identifier to your shipping methods (that is not displayed to the user)? That way we could set the same id’s on both the old and the new website and handle the migration more smoothly.