It happens.  I send fragile items like vases throught the post regulalry and I am always reminded by the LPO staff that they do not offer a fragile service.

Mostly it is successful.  I have very occassionally had an item arrive damaged, even though well packaged - sometimes its just the angle it is on when knocked.

I wouldnt hesitate to refund the customer for the purchase price, and I would think for the sake of $10 the OP's seller would be just as gracious.