A4A: referrals shopping cart update

[andrii-lysenko]

Resolves https://github.com/Automattic/jetpack-genesis/issues/358

Proposed Changes

Update shopping cart while in Referral mode.

Note: commission values are currently hardcoded and will be addressed in the future.

Why are these changes being made?

Testing Instructions

• Navigate to /marketplace/products?purchase_type=referral.
• Add several items to cart and open it.
• Check that UI is correct.

Pre-merge Checklist

• [ ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• [ ] Have you written new tests for your changes?
• [ ] Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• [ ] Have you checked for TypeScript, React or other console errors?
• [ ] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [ ] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [ ] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?