A pre-authorization is a common online practice and is done in order to ensure a credit card is valid before service is provided.
A pre-authorization is a temporary hold of funds. The amount would usually be $15 which would be released within an hour back to the credit card. It may take a bit longer for the bank or credit card provider to reflect the release of the funds.
Kasamba offers PayPal as an alternative payment method for you to use if you wish to avoid preauthorization.