Thank you for contributing to the Clarizen Forum. We do not have an official way to provide an Invoice as there are some legal aspects to make it officially an Invoice. We do, however, have Reports you can use that will show time based on certain criteria. If you go to the Reports module, you can expand the budget reports and see a number of reports that should create what you are looking for. For example, the Customer Billing report may be the one you are looking for, depending on what you want to see int he report or maybe you will require to create a custom report to meet your needs.
Also be aware that you can configure Views to show data as you wish and export them to excel. We even have an Excel Plugin. An excellent query tool that allows you to query your Clarizen data and save in Excel. The plug in has a cost but here is a URL if you are interested...
Last note, our Product team is in the middle of drastically improving the functionality available in the Reports Module. There is no specific ETA available yet as to when it will be available but most likely Q4 2014.