Integrate Shopping Cart With Site & Paypal Merchant Pro

My website a d a m r e e d e [login to view URL] (remove spaces) needs a shopping cart to be integrated with the site, so that I can sell items and have people be able to checkout while still on the website. ? I want the shopping cart to be very simple and easy to use. ? I don't want any major changes to the existing layout, I just need buttons next to the items for sale and a checkout icon in the top header. I would also like a sales agreement form to come up when they are about to checkout, that the customer must agree to before payment is processed. I use Paypal Merchant Pro for credit card processing.