My Client Center question

A couple of my clients have requested adwords campaigns. Up until now I set up seperate g-mail accounts and create their campaign. I charge them $25 to manage their campaign monthly. So they may do $100 worth of ads and pay me $125 total for managing it. I put the adwords on my amex and bill them every month.

Now that I have a few clients that want to do this, I've been considering setting up a 'My Client Center.'

Can I still set this up to bill my card, or do I need to enter individual billing info for each customer? Do I also have to give them access to this area?