I think it depends on what he says on dispute, if he says they are defective then he may get postage refund too. Dont rely on paypal to sort it out in your favout thats unlikely, offer him the 20% or whatever of his money he wants back, minus postage and try and keep it out of a SND on paypal.
It also depends on what you said in listing regarding the postage of returns, did you list any terms and conditions. Basically buyer can say they are all defective or some are and pp will believe them, you have an email as proof he said 10 are no good, so tell him you have that as evidence and give him his money back in full for those, whether you give him the postage back is negotiable but if pushed give it to him. You are not however responsible for customs charging, if that is in your terms and conditions or not pp should side with you on that one, but it is wise to mention it in future for international - yes, I know its difficult to mention everything! and in the end buyer can downright lie and pp believes them.
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