dentist scam

Cl
3.5 years ago I started my braces. Paid everything in full to get a $500 discount instead of paying in installments with interest.
Initially was only gonna take 1.5-2 years. Now its finally done, which took twice as long as they claim, which I am fine with because having perfect teeth is more important to me.
They now said I have to pay another $500 for retainers.
The person who referred me to this dentist never need to pay for retainers. I asked them why. They said its because my friend only did hers for less than one year, so she got retainers for free. 
So they reward people for getting it done fast. Ok. But they "penalty" you for doing it twice as long as anticipated because the dentist's skill is not good enough to finish within the time frame they claimed? 
And they never mentioned the retainers fee in the first place. I thought everything was paid in full already. 
They basically said: you either pay $500 to get retainers or you get nothing. Its up to you.
It was stupid of me not having any contract or signed stuff. I trusted them fully. I feel like I got ripped off and does anyone have any idea what I can do?