Mystery Guild Boold Club Sent unordered books and charged my credit card. When books were returned, they will not issue a credit to my card Hanover, Pennsylvania

I joined Mystery Guild Book Club and provided my credit card for billing purposes.  They double shipped an order and also sent another book which I did not order.  When I contacted them, they sent me return authorization stickers to return the books which I did.  Two months have passed, with repeated requests for them to credit my card for the returns.  They keep sending me the same message, "we need more information in order to process this"  They have my name and account number, plus the books that were shipped to me, in their records.  They need nothing else.  They still have not credited my account.  This is a big ripoff with respect to poor customer service.  I would recommend buying books in book stores or discount clubs where you don't have to deal with stuff like this.