I like the way Google Adwords is set up in where they don't charge you until the end of the month or spending limit. Every other PPC I've looked at though requires a min. of $50 charged to your card to start totally non-refundable. And then when you get close to spending that much they auto-debit another $50 or however high you set it at.
Are there any other ones that have billing set up like Google Adwords? Post-Billing?