It can happen on a number of reasons. We recommend the following solutions:
1) Check out if your card is available for international payments;
2) Check out if your card limit is not exceeded;
3) Try to use another browser;
4) It seems, your bank takes safety measures, going along anti-fraud policy when you try to buy from abroad. Call to your bank to validate the payment.
5) Try to change the method on PayPal (a line "Payment method", below the address). Even if you don't have a PayPal account, you can use this method. Click "PayPal checkout" => "Create an account" => put your card data. The PayPal method takes 1% commission from the purchase.