Custom Metrics are available for Google Ads, Microsoft Ads, and Amazon Ads. For Amazon Ads, however, the following metrics are not supported:
All Conversion Value
So if you were to select in a dropdown a custom metric that uses any of these two metrics in the formula, we will return the number of conversions as 0, and no alert will be triggered for this platform.
Once the calculated metrics are set up on the All Accounts dashboard, they can be used as alerts.
To read more about how to set up these calculated metrics on the All Accounts Dashboard, head over to this article.