>>>but the buyer gave me an incorrect zip code and admitted it in his email.

This can cause huge delays and I sincerely hope that the chip turns up and resolves this issue for you..

However....

>>>Show me any rule where it says the seller assumes all responsibility for the Post Office getting the package delivered.

You accepted PayPal as a payment right? PayPal provides the seller with instructions quite clearly as follows:

***For transactions equal to $250.00 USD or more in value, the seller needs to provide proof-of-receipt (not just of shipment) from a shipper that the seller does not control, which can be tracked online by PayPal representatives. The proof of receipt must indicate that the buyer signed for or otherwise acknowledged receipt of the goods***

You didn't do that and to make matters worse, you did not properly insure the item even though (per above) you were clearly at risk.

If I buy ANYTHING online and it doesn't arrive, I am going to get my money back. Any seller that tries to pass the assumption of risk to the buyer is always going to be in for a rude awakening.