Need help with free shipping option using 3rd party carrier-calculated shipping

[Rudij47]

Hi Team, 

I'm really stuck on how to add a free shipping option while using my 3rd party carrier-calculated shipping app.

I am using Interparcel for our shipping provider and have installed their app. Inside their app, I have set up my shipping rules for different products and postcodes. Some examples of these are:

1. If "product A" is ordered within "postcode group A" then charge shipping and show our selected choice of couriers. This option is working fine.

2. If "product A" is ordered within "postcode group B" then make shipping free. This is not working.

It works inside the app but Shopify is overriding this rule and replacing it with the Shopify backup shipping rates (Sendle) instead of showing "free shipping"

How can I stop this? I want to have it so that if the conditions are met for example 2, the cart shows free shipping.

I can't see a way to to remove Shopify/Sendle. The only option it gives is to create a new rule or zone. I have experimented with these by value and by weight but I can't get them to work.

When I apply any of these conditions and try to checkout using a postcode from my free shipping rule (set in the Interparcel app), I can't checkout. The error message says something like "shipping is not available for this address".

Is there a workaround for this?

Cheers 

Rudi