You may also have to face the fact that some customers’ requirements for invoice submission are so strict that you have to prepare a custom invoice for them, or submit online on their system, or submit on their template, etc. Invoicing governments is often like that.
In such a case, it is probably not be worth the effort to force Manager to produce what the customer demands. Create a standard Manager sales invoice and transcribe the information to a mock invoice in the form the customer wishes to see it (in Word or Excel, for example). You get all the benefits of tracking things through your accounting system and the customer gets what they want. Plus, you get credit for following their instructions.