Should I create a new contract every time I need to send an invoice to my client?

You can create a Project type of contract, a Time & Materials type of contract or Agency type of contract with your client. All types of contracts support multiple invoices. What this means is the following: if your project takes 5 months and you agreed with your client for it to be paid in monthly installments, there will be only one contract related to that particular project but you will be able to issue 5 (or more) contract-related invoices. The same applies to Time & Materials type of contract – you can create a single contract and issue an unlimited number of invoices to your client after a particular assignment is completed, or on a weekly, bi-weekly, or monthly basis, depending on the terms agreed.