Honest question, can't this be achieved with troff/groff and maybe in combination with m4? This is one of the tools on my TOLEARN list as is something that has been forever in the command line tools.
Yes, this is definitely possible with troff/groff/latex/etc... I built this tool as I wanted nice looking invoices that could be generated instantly. I want to add templates in the future so that I can change the style as well. I spend most of my time on the CLI so having interoperability with other tools such as `date` and `time` etc... to combine commands and script the generation process was one of my goals.
Yes, one could use troff/groff to generate a PDF invoice. You might want to try it before you suggest that it is somehow a better way.