在AWS support 开了一个case，描述了事情的经过和我的期望。
AWS的客服效率很高，第二天就给了我回复，大概的意思告诉我，看了我的账户目前还在享受一年的free tier，让我以后注意free tier的使用，最关键的是客服帮助我抵消掉了200美刀的DynamoDB的费用！一下是客服回复部分内容。
This is Nancy from AWS billing and accounts. I hope you are having a good day by the time you receive this email!
I understand that you were working with DynamoDB service and that you forgot to cancel the service which ran two days. Not to worry! I am more than glad to help you to offset the DynamoDB's charge.
However please note that in free tier not all services are covered and only specific resources with fixed monthly usage limits are covered.
You can check the list of free service from this link: https://aws.amazon.com/free/
Please note that if you keep any On Demand resource which are not covered under the free tier as active in your account, it will keep on incurring On Demand charges.
Now we can definitely help you out with your request of a billing adjustment from our end, however in order to do so, we need to ensure first that the resources for which you have incurred the charges are completely terminated so that you do not get additional charges for them.
I can see that at the moment you have already terminated the active resource that were generating charges in your account. Hence, since I completely understand that you were not aware of the charges you were incurring, I'm happy to inform you that I have added a credit to your account for the charges generated by the DynamoDB. This credit will automatically apply towards this month’s usage and can be referenced in your Account and Billing Console here:
To avoid unexpected charges in the future, please make sure to monitor your usage periodically and let us know if you have any questions regarding the billing aspects of our services. A few recommendations include: CloudWatch billing alerts and alarms, budgets, and CloudTrail. For more information on these services and their features, please review the documentation links below:
We would like to politely remind you, as part of our Service Agreement you are responsible for all activities that occur under your account, including all applicable fees, and charges for the use of our services.
I really trust that this information was helpful for you. Our customer's satisfaction has always been my top priority, so I really hope that my service have met or gone beyond your expectations.
If you have further questions, don't hesitate to write us back and we'll get back to you immediately to help you out as far as we can!
Enjoy the rest of your day!
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