Quite quick and to the point, this one really. ITs not 100% guaranteed to work every time - nothing is, but it will greatly help you for the majority of charge-back attempts.
Trackable mail. NOT! Proof of postage!
All sellers have a different mode of operating. I'm going to tell you mine, so you know what I mean. Whether you decide to use my method, or adapt it to suit your own needs, then thats upto you - but this does help avoiding chargebacks. I have had one successful chargeback, and this was prior to my change to sending via a tracked service.
A charge back can be issued by any buyer, who has paid using PayPal - who states they have not received their item.
PayPal expect any seller to be able to prove delivery, when dealing with a customer charge back. Proof of postage is exactly that. It requires no signature, and cannot be proved to be delivered.
This is what I do when I've parcelled/sent my items.
Click my eBay
Click the transaction in question.
Click view PayPal payment
Add tracking information (at bottom of the payment page).
This brings up a few drop down boxes.
Item "In Process" or "Sent"
Enter the correct delivery method. (I use Recorded, Special Delivery, and International Signed for).
Thats it! You've done all you can there. You have provided your trackable proof, to PayPal before they've even requested it. Thats your first step.
The only other thing you can do really now, is have the url of the tracking facility of the country your item is going to. IE: Royal Mail website for UK deliveries, or USPS for America, etc. If you are subject to a chargeback, and are asked any information by PayPal - add this information in even if they don't request it. It makes their job easier, perhaps they'll appreciate you for it!