Pay Attention to eBay Packing Slips
Lost packages are the bane of all carriers since they started accepting packages for delivery.
The Postal Service's Dead Letter Office employees (call them DLOs) are the only people legally permitted to open lost mail. When an address label gets smooshed, torn, wet, or otherwise illegible enough that the box can't be delivered, it finds its way into the hands of the DLOs, who open the package with the hope of finding enough information to get it to the rightful owner.
Make sure that you always include a packing slip -- like the kind you print from Selling Manager or My eBay -- inside your packages. The packing slip should have both your address and the buyer's so that if the label is illegible, the packing slip will identify the owner and the package can be delivered.