Go to your order history page and find the transaction. There's an actions drop-down that will include an option to contact the buyer. You can also click on the buyer's username and contact them from their feedback page.
There are 2 instances where you would be able to deduct a lost dollar amount for the missing club and it appears that you do not qualify for either situation. One is being a Top Rated Seller, and the other is to offer Free Returns. Since you don't fit in either of those categories eBay will require you to refund the buyer in full. Your best bet would be to try and work with the buyer and get the missing club back.
If the buyer does not cooperate and you end up with a lost club then you should report them for returning an item in a condition other than how it was sent - you can find the report option in the same actions drop down described above. You also have the option of adding the buyer to your blocked bidder list to prevent them from buying additional items from you using that ID.
