If you listed in your auction about buyer having to pay custom charge then they should pay but if you didnt then you should pay
IMO not accurate. Regardless if a seller indicates this or not, as long as her auction states she is in Country A and the buyer wants the item in Country B, the BUYER is responsible for customs charges in their own country. In this case the buyer was obviously not ignornant to what customs charges are, they just wanted the OP to commit mail fraud and declare it at a lowe value than it is worth.
People go back and forth on this all the time in this forum, but the bottom line is if a seller states they will declare the full value and the buyer doesn't arrange before shipment/payment that the seller is willing to underdeclare, the customs are the buyer's responsibility.