In case this has not been resolved, I think what you need is (assuming you handle your own billing instead of rolling up the billing to a consolidated account):
Enable AWS programmatic detailed billing. The service delivers detailed billing to a S3 bucket in your account
Create an IAM user to give the S3 bucket read permission
Use the IAM user's key and key secret in [misc] section, corpkey configuration line
If you use consolidated billing, then you need to do this in the payer's account - enable CSV file deliery and create IAM user to read the S3 bucket in that account.
Hope this helps,
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