If you are working on setting-up the app, and you haven't added rates to your Default product group, and you have a bunch of products in your Default group, you may be experiencing an issue, due to not fully understanding how the app works.

Your set-up may look something like this:

In this set-up, a product group Heavy Items has been created, and with a $30/item rate added to it.

The Default group, which has still has the majority of products in it, has no rates added.

The goal of this set-up would be to have a $30/item ship cost added to whatever the regular/pre-existing shipping rates are.

However, it is important to understand that:
The app cannot modify, edit, adjust, control, influence, or hide your pre-existing rates.

So, the app cannot add $30/item for the heavy items to the already shipping rate.

Let's take a look in the Shipping Settings:

With this set-up, where there is a native Shopify rate, and the app connected. The app cannot tack on a price to the $12.00 Ground Shipping rate.

Instead the app will just show rates based on the logic of the app, and the Ground Shipping rate will also show no matter what.

So, with this set-up, here's what it looks like when there is only a product from the Heavy Item product group in an order:

The app can not stop the pre-existing rate from showing.
The app shows the $30/item rate because there is only one product group in the order, and that product group has a rate assigned to it.

Now, here is what it looks like when there is a product from the Default group, and a product from the Heavy Item group in an order:

The native Shopify rate of $12.00 shows, because it will always show and the app can't stop it
There is no rate from the app because the Carrot in the order belongs to the Default group, and there are no rates set-up for the Default group, therefore the rates from the app fail.

So, this means you should re-configure your pre-existing rate into the Default Group

So now let's reconfigure the native Shopify rate Ground Shipping for $12.00, into the Default group in the app.

Now let's compare how the rates show after the pre-existing rate has been re-configured within the Default group.

Because there are rates set-up for both the Default group, and Heavy Items group, a blended rate from the app appears.
The pre-existing Shopify rate still also appears, so you should manually remove the rate from your shipping zone (after you switch the app to Live Mode ) now that the app is handling the rates for all products.