The service team sent back a bunch of general information about costs and instructions to remove a "Metric Filter" which from my follow-up I can confirm does not exist on my account.
The detailed billing shows charges like this across US regions:
$0.00 per request - first 1,000,000 requests 239,785 Requests
$0.01 per 1,000 metrics requested using GetMetricData API - US West (Northern California) 295,570 Metrics
Do others have experience with miscellaneous charges that are impossible to cancel? Any tips for dealing with support on this to get answers beyond copy/paste support articles?
In your case, I'd guess that some part of your system, or perhaps some integration that you added to your account, is making API calls without you being aware of it.
After much investigation I realised that one of my dev has setup complicated stuff using some Terraform config he found on Github.
I feel that AWS has very bad UX.
Maybe try the aws cli to list and delete?