While PayPal could be integrated with the ecommerce package I wouldn't recommend it for most sites. PayPal has a very limited API which is fine for basic transactions. That is a transaction for an order that doesn't require modifications once the order has been placed.
E.g. payments for software downloads fit the PayPal feature set well. The visitor pays for the license and receive access to the software download area.
Orders consisting of several physical items that need to be shipped and could be out of stock are a different matter. The consumer might also want to change the order before the items are shipped.
Take a good look at the PayPal API before making a decission. There are other credit card gateways that might fit your needs better. And not every one likes to setup a PayPal account just to pay for their order.
The ecommerce module requires quite a bit of work to interface to another payment gateway then CyberCash. I am currently working on a gateway to authorize.net. CyberCash had a unique workflow that is different from every other payment gateway that I have looked at. The current ecommerce module doesn't handle returns and voided items well either. I think that this is an issue that justifies a team approach. Any one interested?
