This has been submitted as an idea for future solution. If you have not already voted on it. Go here to do so. I need this functionality as well. https://community.servicetitan.com/t5/Ideas/bd-p/ideas
I increased our prices by 3% across the board. then I set up a ST membership called "Cash discount". My field techs ask if the customer wants to get a 3% discount by paying with check. If they say yes, then we add the "cash discount membership" to the invoice and it takes 3% off the total.