I am looking for the same thing. There seems to be some confusion. There are third party apps that attach documents to the invoices in bulk. I am not sure if they also check the checkbox for you. Quickbooks currently neither allows a bulk attachment upload, not does it have an option to click the checkbox by derfault. Both need to be changed. But the one of having a default with the checkbox checked is a very easy thing that quickbooks should do right away.