CarrierService Rate Show Discount/Savings

Shopify Partner
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I'd like to be able to show the customer how much money they are saving when a shipping discount is applied by a CarrierService app. Is there any way of doing this? It doesn't appear that it is possible on the checkout page (can't really customize much on a non-Plus store), but it seems like it may be possible on the cart page using an AJAX shipping calculator.

Examining the AJAX response from the /cart/async_shipping_rates endpoint (which @carolineschnapp's shipping-calculator pulls from), there is a field for compare_price, but the CarrierService documentation doesn't describe any way of populating that data. Using Shopify's built-in carrier-caluculated shipping, however, that field does have a value (presumably the public, non-discounted Canada Post rates).

Is there any way to provide that data using the CarrierService API?

If you look at the data below (truncated for clarity), the first quote is from Shopify's live rates and does have a compare at price, but the second quote is from a shipping app and does not show the markdown (the second quote has free shipping applied on the app-side).

    "name": "Expedited Parcel",
    "code": "DOM.EP",
    "price": "$29.71",
    "markup": "0.00",
    "compare_price": "34.60",
    "source": "canada_post",
    "name": "UPS® Standard",
    "code": "ups-STD",
    "price": "$0.00",
    "markup": "0.00",
    "compare_price": "0.00",
    "source": "CarrierServiceApp",