We first recommend you make contact with the customer via our messaging system and clarify the situation.
Then after discussing it with your customer, you will need to respond to the dispute via PayPal's resolution process.
This can be done in your PayPal account. If you can mutually agree on a solution and respond with the agreed solution via PayPal this will make things more straight forward.
Do not stress this does happen from time to time and simply deal with the matter respectfully, generally speaking it simply comes from a misunderstanding in communication. So keep things simple :)