AWS Billing Alert
It is a strongly recommended that you setup an AWS Billing Alert (aka AWS Billing Alarm) and frequently check your My Account tab Account Activity on your AWS account to verify that you are not exceeding the Free Tier. Under the Details section of the same screen, you can see how much of the Free Tier service you have used for that month. A good thing to do until you understand the limits and how they relate to the Free Cloud Desktop.
An Amazon Web Services (AWS) account is used to power the Free Cloud Desktop. While setting up your AWS account you must provide Amazon with a credit card. You need to actively monitor and manage your AWS Account Activity to ensure that you are not being charged for something you didn’t want.
For the AWS Free Tier, you can set a billing alert (Billing and Cost Management Tab) to notify you if you exceed the free tier by setting a threshold of $0.00. Below is a link to that information (you may need to be logged into AWS to view the page). Also, it is a good idea to monitor your account frequently, at least at first to ensure you are running Free and not approaching the limits of the Free Tier for the month!
You must be already logged in to your AWS Account for these links to work.
Here is more information on the AWS Free Tier.