Inter Service Esthétique
The app seems essential but does not work with the Ella theme. Maybe the problem comes from the cart in ajax. You should also be able to apply the rule to all the products in the store without selecting them one by one.
We are more than happy to take a quick look at any theme issues. Please shoot us a note about your theme issues (email@example.com). Although we do test on over 20 themes on a regular basis, sometimes newer or paid themes work differently as they are all built by different developers.
In terms, of product selection, you can select up to 200 products at a time with one click and then go to the next page to select another 200. We find most of our clients have different rules/fees so they end up hand selecting anyways.