It is possible to generate an invoice from a project which is based on the time spent on a project, as recorded in activities linked to it. 

This is similar to the function within Helpdesk tickets, and works as follows.

A default product for time used is specified, a price is specified and the time increment this represents i.e. per hour, half hour etc. Relevant settings for this are found in Additional Settings under the category "Ticket/Project Invoicing".

When clicking invoice from the project the system shows a list of linked activities and the duration for each. Click and select the activities to invoice.

When creating the invoice, the system totals the activity durations and the price is applied as a single product row using the product specified in the settings.