Shopping Cart - Checking for Existing Web Store

We have a shopping on our website that includes a product catalog and products pages. As of now we Paypal's shopping cart - checkout. We need our own shopping cart so we can add a merchant account and still keep Paypal. What we need is to have our customer go to our product page then add a product that will go into the new cart. From there the customer can add more products ,delete a product, delete all cart contents etc... From there we need a checkout page that will ask for the customers billing and shipping address. Once all this info (Shopping Cart Contents, Billing and Shipping info is taken the customer is presented with 2 payment options. Paypal or Net1 Gateway(credit cards,e-checks.) Once the customer selects their prefered payment method the pre-populated data to send to either Paypal secure site or Net1 Gateway secure server where they add their creditcard data there. A receipt is sent to the customer once they leave our site for their records. We don't need any Paypal IPN as we already have it. Just a simple straight to the point cart with checkout. Hope this makes sense.