Fisherman has it right. Send all info to paypal, work with them to the best of your ability.
When you make your invoices, do you write out a Terms of Agreement? I always do this with my invoices just to be safe (return policy, and if damaged/dead/broken ON ARRIVAL, PM me on geekhack/wherever I sold the item); have not had an issue but may be something to consider in the future should something need to be looked at via PayPal.