It´s report definition file (.rdlc) ..\Shared\Apps\Reporting\Reports. Open the "InvoiceRecieptReport.rdlc" in VS and change it in the editor.

I already tried that but I would like to add a custom MetaField (CustomerContact object) on the invoice report. How can I do that, is it necessary to change the data source of the report?

You mean a custom field that you have added to the contact class? The name of the customer is already in the dataset.

I had a look at it and tried to fiddle directly with the xml in the report .rdlc. Additionally, VS doesn’t behave well with the report, I didn't get the results I expected. You probably have to have a look at changing the data source.

Any idea, how to configure a new data source for the report? If I decompile the dll's it seems that the data source is hardcoded in the source.

Did anyone find a solution to this? and how they approach it?

I guess you have to ping the Commerce dev-team about this.

Our official answer for customizations made to Commerce Manager is it's not officially supported. 

I personally can see how to do it, and I might write about that if time permits, but if it's urgent you should turn to expert services for help.