Legal Question in Business Law in California

It may constitute fraud if the person sending the invoice has knowledge of its falsity. Your question begs many questions in return.

This is what we call a loaded question. Instead of telling us what happened, you've offered your own characterization. I might see things the same way if I had some facts to go on, or I might not.

Whether the invoices are legal is a different question from how you feel about them. It depends on what happened and what they say. Without that information, we can't guide you.