Add E-com Functionality in existing website

I already have a fully-designed website in place; what it lacks is the ability for visitors to place items in a cart and check out. The site will be used to sell physical products only, so inventory needs to decrement as items are purchased and the order details must flow straight into my existing order-management area (built in PHP/MySQL).

My immediate priorities are:
• A seamless shopping-cart experience that matches the current look and feel of the site.
• PayPal integration as the sole payment method at launch, with the code structured so additional gateways (credit/debit cards, bank transfer) can be bolted on later without a full rewrite.
• Basic tax and shipping calculations (flat-rate is fine for now).
• Confirmation emails to both customer and admin.

Please deliver clean, well-commented code and a short hand-off guide that shows me where to adjust product data and shipping rules in the future. If you have worked with PayPal’s latest REST APIs or smart buttons, let me know; that’s the direction I’d like to go.