I have created a free tier account in GCP for learning purpose based on the assurance "New customers also get $300 in free credits to fully explore and conduct an assessment of Google Cloud Platform. You won’t be charged until you choose to upgrade."

I haven't upgraded my GCP to a paid account, But I have been receiving hefty invoices in Mail. Could anyone help me in this regard. I could also not find a email or a phone support, so that I can enquire regarding this