Hi swm023,
Are you able to create an invoice for any other existing customer in Method when you don't select the checkbox for "To be e-mailed in QuickBooks?"
If you are having trouble creating the invoice for this customer when you place a checkmark in the "To be emailed in QuickBooks?", are you getting the following error message?
You cannot mark this 'To Be Emailed' since your company does not have a valid email address entered. Go to QuickBooks and click Company > Company Information, and enter an e-mail address.
If that is the case, can you please follow the instructions mentioned in that error message. In addition, please conduct a "Changes-Only Synchronization" and see if that changes anything.
I hope that this helps.
Ronen