I think this would be a great ap if you didn't need "carrier" services to run it. Maybe I missed it but i'd love to have been told that before I got it all set up and realized because I didn't have the most expensive shopify payment plan that I don't have carrier services, which in turn means I cannot use this app.
At first I thought the app was corrupt, but after enabling the CCS feature, it worked perfectly fine.
You also need to import your predefined shipping settings to the App, then delete these settings from Shopify.
The only missing feature I would say is the Search or Filter by Tags in the main menu, all of my "Free Plus Shipping" items have the tag "free", so having the ability to search products by their tags would be a huge benefit.
Other than that, works perfectly fine.
BTW, you don't have to upgrade your plan to enable the CCS feature, you can ask Shopify to enable it and they'll add $20 to your bill.