How do I ensure that an invoice is only sent to a company?

In the financial menu the different payment methods can be found. The method will be determined based on the customer level. If the option 'email' is chosen, then the customers email address will be used to send an invoice.

This is applicable to 'Send email' as well as 'Completed'. In case the company has an internal sending method, 'Do not send' will appear under the customer details. Then the company's own sending method will be used.

Do you want to distinguish, within a single customer or company, between the address for the reservation workflow and the address for invoices? You arrange that with the Workflow and Invoicing check boxes in the Contact details block. See How do I set a separate e-mail address for sending invoices?