Proceed with caution when using "zero dollar authorizations" with auto orders (recurring billing). Even though it seems like a ideal way to offer a trial via an initially zero cost, very often it will create customer service issues because the customer will often confuse the temp hold as a actual charge which may result in customer service headaches. And as is the case with auto orders in general, you may need to take precautions to block prepaid and gift cards to prevent abuse. It's generally better to do a minimal transaction ($1-$5) up front in most cases.