When you are setting up your product/service or event you will see that there is an option for Offline or Online payments.
Online payments means that your clients will be auto directed to a payment portal ( whichever you have linked to your account) and will be required to pay for the service at the time of booking.
Offline payments means that clients can place a booking for a paid service without paying at the time of booking. It is then up to you as how you would like to accept payment from your client. ( invoice them, accept cash in face to face meetings, bank transfer etc.) You can include info about this in your success message text or in the service description if you would prefer.