The buyer took a risk buying used shocks that were not stated to be tested and in good working condition. And to request a refund that long after the auction is just stupid. He's upset that he took a risk and got burned, and now wants to blame anybody but himself for it.
The extra cost he had to pay to pick up the package was probably customs fees or something along those lines. If it was in fact and error on your part and you didn't pay enough postage, he should have contacted you before accepting the package to see if you would pay the extra fees, and if you declined he should have refused to accept it. By picking up the package and paying the fees before talking to you, he absolved you of any responsibility for paying them.
The ebay system will allow feedback to be left up to 90 days after the auction end, but there is a way to leave it long after that if you still have the auction number. I would tell him up front that I had no intention to refund any amount to him and that any problems with shipping or condition of the item should have been resolved weeks ago. If he leaves negative feedback, leave a calm, factual response and any potential buyers in future auctions will be able to tell who the real idiot is.