Enhancement: How to create button on Payment page (for tax id or general discount) 

Thanks for this js! 
I'm a Python dev and js is not in my skills.
Is there the possibility to create a new button on POS payment page? 
It would be very useful for things like tax id that you don't need to store (if you don't want it, obviously) or to trigger a discount before validating the order. any idea?
Thanks!