If you are saying that someone made a claim against you and stated they did not receive a product and paypal withdrew the money, then this is due to you not providing them with enough information or the correct information.

If you are selling digital products online, you need to make sure you use a system that makes the buyer log in, and has the downloads available, and logs the ip, username and possibly even a registration code sent to them in an email.

We have created and use various systems see more like this, as paypal will want proof of delivery, and just sending them an email is not enough. (since these can be faked, and so forth)

CyberXing