Support quantity in subscription checkout flow (Default UI)

🔍 Problem Statement

Pelcro's cart functionality only supports one-time ecommerce product purchases. Subscription products cannot be added to a cart, which means customers cannot bundle subscriptions with other items in a single checkout — limiting cross-sell and upsell opportunities.

💡 User Story

As a subscriber, I want to add subscription products to my cart alongside one-time purchases, so that I can complete a single checkout for everything I want to buy.

As a principal, I want to order a large and exact number of subscriptions at one time, so that I can buy magazines for entire classrooms.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a cart with both subscription and one-time products, when the customer completes checkout, then all items are processed correctly — one-time charges are invoiced immediately and subscriptions are created with their respective billing cycles.

1. Have quantity in the checkout flow for one product/plan.

2. plan price x quantity

3. with same address