Many government agencies have to go through 3rd parties ("lowest qualified bidder", which AWS often doesn't bid on) to pay our bill... And the contractors themselves use 3rd parties to figure out how much to bill us... so things like AWS bill alerts are not possible (imagine the whole billing section being permission denied while using the root account user). In addition, 3rd parties do not always provide great tools to set up bill alerts.
We have 40+ AWS account and can't track our spend without 10 button clicks per account which can't be done in parallel due to the apps browser caching locking us to working with one account at a time or it corrupts the cache. The process takes a minute per account, if the website doesn't crash trying to process our records (which are >3 million line item records for some accounts)
All that said: we basically were screwed at monitoring our bill.
In the fun that is semi-server less, we had a container running in ECS and logged directly into cloud watch. It went into an infinite loop late one month. It didn't really take our bill out of the expected ballpark when it was processed by the third party and delivered to us 20 days after the bill cycle closed
Next month however, by the time we got the bill, it had run for the ENTIRE month in an infinite loop, and already 75% of the next month. It pushed 50,000GB of an infinite loop log data at 50 cents per GB ingested the first month (plus storage costs). (That's about $30,000 for month 1)
AWS did not provide assistance because it was paid for on credits, but it basically ate all or remaining credits after the second month bill came in.
After that, we got the contractor to simply give us a copy of the "detailed billing reports" daily and built a process to make our own bill monitors (which at that stage had markups from the contractor). We eventually got somewhat of a better monitoring system through the third party app as well, but we were not aware that was possible because it was not accessible without the contractor setting it up for us (hidden menu options to combine accounts)