A number of my product variations require a more expensive shipping service than others. I want to be able to offer a different flat rate shipping service for those selected items. I would like to be able to use the conditions on a particular flat rate shipping service to restrict its appearance to only those carts that have those particular product variations in them.

I believe that to achieve this, I need to work with the Rules configuration to draw in the appropriate entity fields. Unfortunately, despite the numerous similar questions and the screencasts that get close to the issue, I can't seem to make it work.

I think that what I need to do is:

Conditions > Entity has field (select the product field that determines whether or not a higher shipping rate is required)
Conditions > Data comparison (check the value of that field)

Unfortunately, when I am selecting the 'Entity has field', I can only choose between
"site:..", "commerce-order", and "commerce-order:...". I can select the field I require, but when it comes to the 'Data comparison' bit, I can't select the field in the data selection.

Am I going about this the wrong way, or is there just something I'm missing to get this working? Any help would be much appreciated!