Is there a way to programmatically add a field + value to an invoice HTML or PDF template? Right now I'm thinking about adding the field to the template even if the value is empty, doesn't look the best though.

Hey, I hope someone answered this for you. If not, let me know. If I understand your question correctly, this can be done using freemarker on Advanced PDF templates

I am not sure what are you trying to do @twandy , but for example I use mustache library for crearing invoice templates.