Please ask support team to activate Billing Contact feature for your subdomain. Once activated you will be able to see a 'Billing Contact' column in your settings under 'Customer --> Contact Types'. Add a contact type and set it to Active and also as Billing Contact.
After creating a Contact Type with Billing Contact turned on, go to the customer for which you want to set a contact as Billing Contact. Click on Contacts tab and select a contact, this will open a panel to right side with a list of contact types. From this list select the billing contact you created and Save.
You can set as many billing contacts as you need from available contacts list. Once billing contacts are set then while creating an invoice from a work order for the customer for which you have set billing contacts, you will be able to choose a Bill To contact as shown in below screenshot.
Printed invoice having billing contact looks something like below screenshot.