Note also that - in respect of the initial free trial - whenever payment information is added or updated in iTunes Store, App Store or Mac App Store, an authorisation request may be made to your card issuer.
This is not an actual charge, just a temporary check to ensure that your payment card is active and able to be used when paying for your transactions. Any such charge will be removed automatically from your card account and will not need to be paid.
More information:
About payment card authorization holds in the iTunes Store - Apple Support