Billing Alerts
Open Billing Alerts in the sidebar, or the Billing Alerts page.
The page describes email alerts when one Pay-as-you-go service renewal invoice exceeds your threshold. This is a service-renewal invoice threshold, not a cap on all monthly account spending.

Choose Enabled, notify me when renewal invoice for my service will be higher than: and enter your threshold, then click Save Changes. Choose Disabled, do not notify me to disable the setting. Check that the saved option and amount are retained when you reopen the page.
An alert does not stop a service or impose a spending limit. Review service costs and invoices separately. If the form does not make the threshold currency clear, confirm it with billing support before relying on the amount.