You're actually discussing three different cards here -- credit, debit and funded accounts --
There isn't really any distinction between a credit and a debit transaction online, there's no pin number to key in so the transaction is still a card not present and should use the CVV2 on the back of the card for verification -- and merchants are not allowed to store the CVV2 numbers.
Pre-paid, funded cards are different BINs than regular bank issued card numbers and those can be blocked by database.
Authorizations cannot lead to a chargeback, as there was no transfer of funds on a capture from the consumer to the merchant. There are many ways to handle free trial situations, some more effective than others. Authorizations that are not captured within whatever time frame is in the contract simply disappear.
Last but not least, you can't disclose that you are going to charge someone 50 bucks and then turn around and try to charge them 40 bucks because the card wouldn't bill or rebill for the entire 50.
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