HTML/CSS E-commerce Storefront Build

I need a clean, fully responsive e-commerce site coded from scratch in plain HTML5 and CSS3 (no WordPress, Shopify or page builders). The store’s core requirement is a robust shopping-cart flow that lets visitors add, edit and remove items seamlessly across desktop and mobile.

Key features I must see working end-to-end:
• Dynamic shopping cart (add/update/delete, quantity change, subtotal update in real time).
• Checkout flow that sends the total straight to PhonePe (UPI) for payment; no other gateways are required right now.
• Product pages and a simple catalog layout styled with modern, lightweight CSS—feel free to use Flexbox or CSS Grid, but no heavy frameworks.
• Clear, reusable HTML/CSS structure so I can later bolt on user accounts or advanced search if needed.

Deliverables
1. All source files: organised HTML, CSS and any JavaScript you write for cart logic.
2. A brief README explaining file structure, how to add new products, and how to update the PhonePe credentials.
3. A live demo link or zip that I can drop onto my server and see working exactly as promised.

Acceptance criteria
– Cart must persist while the visitor navigates between pages.
– PhonePe payment should redirect correctly, return a success/failure response, and display the proper message to the user.
– Pages must score 90+ on Google Lighthouse for performance on mobile.

If you have experience hand-coding storefronts and integrating UPI gateways, I’m ready to start right away.