Shipping Labels: display the discounted price rate in Carrier and Rates screen

[pmusolino]

Fixes #5156

Description

Previously, in Carriers and Rates, just on iOS, we were displaying the retail_rate instead of the discounted price rate. This creates misalignment problems between iOS and Android.

Testing

1. Make sure that your test store has configured the WCShip plugin.
2. Select an order eligible for creating shipping labels.
3. Select Create Shipping Label, configure Ship From and Ship To.
4. Select a package.
5. Proceed to select Carriers and Rates. Notice that the price listed, it's the discounted price (if available) and not the retail rate. The prices between Android and iOS should be aligned.

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-woocommerce]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-woocommerce]

You can trigger an installable build for these changes by visiting CircleCI here.

[itsmeichigo]

LGTM