At the moment Flobot addresses the invoice to the default account owner. This might not be the relevant person to send the invoice to for all orders or all sites held under the account. Eventually, it might be an idea to enable the user to select ho to address the invoice to from a drop down box using data held in the account holder section.
But there are so many variables with email addresses that the simplest solution must be to add a text box for the user to override the default account addressee with a name of their choice.
See a mock-up below