If you do not know the time (e.g. hours) you will spend on your project and you are not sure how to bill your customer, then are two ways for solving this:
1) Create a "Time & material" type project where the amount (e.g. hours) is "0" by default. Once you have delivered the service, you will be able to issue the invoice to the customer with the actual quantities (e.g. hours of work) and price (e.g. hourly price) defined.
2) Retroactively create and submit a "Fixed price" project to the customer once you have delivered the service already and know the time you spent on the project.
Note that both approaches assume you trust your customer enough to start delivering your service without the amount defined in the formal agreement.