Invoicing for Multiple Increments of Work

I'm new to PPH and just got hired by a client for a project. My accepted price is 10 euros for 10 minutes. The project (transcription) will be approx. 120-180 minutes. How do I bill for this so that the entire amount will be in escrow, not just 10 euros? Does the client have to accept the 10 euro offer 12-18 times? I'm guessing not. Just not sure of the procedure, as this is my first PPH project.