Dayum. I'm insured to the teeth, and that happened to me at a spinal surgeon. There by GP referral for janky disks. Made the appt anyway, went in and read the cashier the riot act, not quietly, with a full waiting room in audience. The practice manager appeared, and they saw me without advance payment.
Sometimes a public shaming is a motivator. I was very fortunate. They could have easily called security, but I think my argument was compelling, and it was easier to quietly waive advance payment.
2015. Do not advise now.