they will likely contact you and tell you to return it, ask you to provide tracking, and then refund you. just had the same thing happen with a pair of boots that turned out be a dish sponge...
edit- yes you will need to pay to ship a coke bottle to china or wherever.
Seems doubtful for a coke bottle, rather than a normal condition/measurement issue, but perhaps possible.
He may just be required to destroy the coke bottle.
Paypal's policy is that buyer foots the bill on return shipping, are you sure the pre-paid label wasn't at the seller's expense?