You get your own unique qr code too, so they can just scan it with there phone and it takes them directly to your sign up page.
I just printed off a sheet of paper explaining how payment was going to be taken moving forward, told them to pay the windows by bacs this time, but be signed up to this before next clean.
It's a win, win situation. They sign up, then you get paid. If they don't, then you drop them. They are the ones who's being unreasonable in this situation. Even though you feel like you are, when really, you just want paid for the job you've done. There's nothing unreasonable about that.