As a seller, it is always best to cover all bases and send your artwork with a delivery or signature confirmation. if you print your labels at usps.com with delivery confirmation, it's free and will ensure that can see when the package is delivered to the buyer.

A newer scam that is showing up with art sales lately is for buyers to wait a few weeks and then claim that they haven't received the items they paid for and then they go and do a chargeback at Paypal. If you have records to prove that you indeed did send out the item, you will have much better chances of resolving the issue later.