https://www.paypal.com/us/webapps/helpcenter/helphub/home/
If you've already been there and CS is closed, call them tomorrow. They'll walk you through the process.
Basically, they may be willing to dispute the chargeback on your behalf. That doesn't mean they're going to win against the cc company, and if they don't, you don't either. Meanwhile, the money will be taken out of your account or backup funding source.
If this buyer did the same thing to a lot of sellers, PayPal will be more likely to try to dispute it with the cc company. If you're the only one, you're probably out 12 bucks.
If you had a brick and mortar store, you'd have shoplifters (and still have chargebacks ) - it's annoying as heck when it happens, but you figure it into your cost of doing business and you move on.