I use Google Cloud Platform to host my websites. This is how I make sure the platform stays free and investigate the costs that appear.
Billing => Budget & Alerts => Create Budget => Set Amount to 0 => Finish.
Billing => Reports => See which project has costs.
Billing => Transactions => See description of the cost.
In my case it was “Cloud Storage Standard Storage Europe Multi-region: 0.217 Gibibyte-months [Currency conversion: USD to CHF using rate 0.961]” with the artifacts from building the code before each release.
Storage => Browser => *.artifacts.* => Lifecycle Rules => Add Rule => Age => 1 => Action => Delete => Save.
Storage => Browser => *.artifacts.* => Select all folders => Delete.