This type of applications use templates for the invoices and standard labels which means you will have to modify the template in order to use the desired font.
I've searched the web for additional information related to template modification, but there isn't anything available which means that you will have to get in touch with the developers and submit a ticket on the corresponding support board.
Use the following link to open a tick and ask how to modify the labels, invoices, templates, etc: http://support.avanquest.com/support/tickets/new