Code review comment for lp:~sebastien.beau/e-commerce-addons/e-commerce-addons-improve-invoice-export

Hi I just split the function in two and commit it directly after pushing it.
This avoid in case of error to send twice the invoice. It will be better to add the reporting system on the invoice export, but for now I don't have the time. And I prefer to focus on 7.0 version. So just add this change to make the 6.1 more stable.

Thanks