I keep getting an error message when I try to buy in-app currency. I get prompted with "Contact iTunes Support to complete this purchase," but I don't want to pay $20 to call Apple to tell me how to fix it. Any solutions?

I am trying to buy Fanta Points in a game called Fantasica, however, when I try to make a purchase, after I am prompted to give my Apple ID and password, it gives the message, "Please contact iTunes Support to complete this transaction." I don't understand why I get this message, because I am able to make one purchase, but when I try to make another, that is when I'm prompted and am no longer able to make any purchases for several days. Is there any way to fix this without me having to pay the $20 cost of calling iTunes support?