The most common reasons for denied credit card payments are:
1) the credit card has insufficient available funds,
2) wrong information was typed, especially a wrong zip code is a very common mistake,
3) the credit card is set to work only in a specific country (or countries).
The easiest way to avoid this issue is by selecting Credit Card payment in the payment methods window. Once you see the form for entering your card information click on the PayPal Checkout button instead. Select “pay with debit or credit card” and fill in your information.
Note that this does not require you to have a PayPal account!