Nogdog's suggestion is a sensible consideration. However you might loose sales and clients don't like that.
Another option is to process the credit card immediately using a 3rd party service like Worldpay and if approval fails, provide a refund. Problem is customers don't like that, but then who cares you just rejected them anyway, lol.
The very fact that you're here asking this question lead me to think that you really really ought to consider very hard if you're out of your depth. If it all goes horribly wrong, someone will be held accountable and I'll bet you can guess where the buck stops. If you've accepted this contract as a sole trader it's you personally legally and financially and the punishment can carry a jail sentence. I would hope you've set yourself up as a limited liability company and employed yourself and have insurance. This kind of thing is really heavy duty. It would be better to lose a client then be held accountable for any failure.
Are you hosting the service for your client? If so that's a whole other set of problems. You have to stay on top of security for both your scripts and hardware, and it's not something you leave to a shared hosting company. I don't think there are any companies that store CC detail that don't have their own servers in house. If anyone knows different let us know coz I don't want to use their services!