You won't be charged for any declined purchases. However, be aware that any time a credit card transaction is attempted, the initiating bank will place a temporary authorization hold on the funds while waiting for the transaction to post to your bank account. This is true whether the transaction is approved or declined.
Temporary authorization holds typically expire within 5-7 days, but this depends on the policy of the initiating bank. After the hold expires, the funds for the declined purchases will be returned to your bank account.
There are a number of reasons a transaction might be declined. Please ensure that you are entering your credit card number and expiration date correctly and that your card is not expired. Additionally, be sure that you are entering your billing address exactly as it appears on your credit card statement.