As far as I can see you have only bought one set of strings.  Purchased on:
08-Dec-12 15:48:40

The system is convoluted only if you decide to pay with your CC, which you obviously did.  If you used PP = most recommended, then the payment would be automatically noted and you would not be getting demands for payment, which by the way are automatically sent by eBay.  Contact the seller and tell him to mark your item as paid.