You may handle several clients or projects and wish you could control them all in one place, but split billing and accounting. That would be nice. But, there's something that's a distant second: consolidated billing and multiple accounts. The upside? If the client leaves, you just disconnect them from your (billing) central account and they get to keep their S3 bucket(s).

Set up the unique S3 or AWS account, set up the S3 credentials and then log into your centralized account and configure Amazon's Consolidated Billing mechanism. You'll receive itemized billing with details like bandwidth and costs for each individual account. It makes for a pretty clean setup. In our case, we use a unqiue email address for the account to reflect what job it belongs to and the client need never be bothered. It works equally well for multiple projects that need their own S3 reporting.