Hi Marcelo,
This actually wouldn't be too difficult, we first need to add a link to the email address that is stored for your Customer, in the Customer table, then you should be able to access this field from your Report Designer to add to the Report.
First, go to Customize->Tables/Fields, locate the Invoice table, and click on "Edit Fields" for this table. Now, at the top of the pop-up screen, choose the option for "Add a Linked Field", choose the Customer table and Email field, then if you want, rename the field however you want for the "Display As" area. From here on out, if you have data in the Customer table for the email address (this is the default email field for the Customer table), then it will be stored in your Invoice table in your new "Customer Email" linked field.
Screenshot of Creating Email linked field in Invoice Table:
To prove this, after adding the "Customer Email" linked field to my Invoice table, I then opened up my Method Report Designer and pulled up the SimpleInvoice Report. Once it was loaded, I shrunk the "Description" field on this invoice in order to make room for my new "Customer Email" field. I mimicked how the other fields were added to this invoice, first adding a label with the caption "Customer Email". Then in the next section, i dragged over the "Customer email" field found under the "FieldList" area on the top right of the Report Designer, grouped in the "Invoice" table.
This was it, I then saved my new Report, made sure my invoice screen would pull my new invoice when I clicked the Print Preview button on the Invoice screen. I took a screenshot of my Invoice Report after making this change, take note of the Customer's Email address now included on this Invoice Report:
I hope this helps, please let me know if you have any further questions.
-Ben