I recently bought a shoe site that was using primarily PP for checkout, being an ecommerce site I would lose sales when the final checkout (paypal) popped up. I applied for merchant pro and was approved so know I need advice on which shopping cart that is preintegrated with PP merchant pro is easy to use.
http://solutions.paypal.com/procarts/ Here is a list of their shopping carts. What system are you using right now? If you are using a popular system, it may be possible to integrate with what you have.