"Free Shipping" line triggered in Order Status Page when automatic shipping discount applied

We use the BSure app to create partial shipping discounts. The app itself works perfectly at checkout- no problems there. The problem is when the order is completed.

On my demo customer account, I go to my account profile and view the Order Status page. The final discounted shipping value is correct, the name of the discount is correct - but it triggers Free Shipping text on the discount line.

Shopify Support say that they can’t do anything about it, and that:

“…because the discount is coming from the BSure app, Shopify’s default checkout logic is mislabeling that 50% off.”

Does anyone know how to edit “Shopify’s default checkout logic” so that the Free Shipping line shows the actual discount value?

Thanks!