I’m pretty sure Kaiser Permanente will bill you, and hound you if you don’t pay at time of service.
It’s part of the negotiated rate for group insurance. My sister and I both have Kaiser. She has a $10 copay, mine is $20. Because they premiums and copays were negotiated by different employers.
I guess you could not pay it – but they could come after you for it. Perhaps your doctor decided the cost to bill is more than the copay… but if enough people do it – they’ll come after you.
Just curious – do you pay your deductible if you’re in a car accident and get your car repaired? Or is that optional also? Do you pay the full ask when you get a haircut – or do you pay a lesser amount and hope they don’t mind too much?