I always have the clients setup their own Google Billing account, tied to one of their email accounts, and using a payment card of theirs. I have actually used “prepaid cards” in a few cases for this (these cards may have little to no actual money on them…). I explain that Google gives $200 credit per month, which translates, at least on the basic maps API to 100,000 desktop views per month (mobile is free) and unless they become an extremely popular destination, they will never come anywhere close to this!
But this also leads into using this same Google billing account to be used for other services like Google Adwords, which most businesses are interested in trying, even on a small scale. Gives you a nice way to have an upsell, ongoing product to offer!