I sold Super Mario 2 for NES complete with box and instructions. I charged the buyer $5 for shipping via USPS. The buyer takes it upon himself to add an extra $2 (total $7) to have it shipped in a box rather than a padded mailer because he wants to make sure the game's box stays in good shape during transit. He received the package and notices that the actual postage cost only $2.66 and messages me via ebay that I overcharged him for shipping and is "hoping" to get a refund. How should I handle this?