Ignore a $5 medical bill?

Last year, I went to see a specialist. The receptionist told me there was a $15 copay which I paid.

I did suspect that the copay should have been $20 but thought that maybe I was mistaken so didn't bring it up.

A while later, I received a bill for $5. I thought to myself, let's see how many times they send this bill to me that is costing them 40 cents to mail each time.

Fast forward a few months and the bill is now 120 days overdue and they have written "Your account is seriously past due. Please contact our office today to avoid collection action".

That was on a statement dated mid February and also on a statement dated mid March.

On principal, I don't feel I should pay. Why? Because they made the mistake of not collecting the correct amount when I paid the copay and so why should I now spend time mailing them a $5 payment.

But of course, there is the danger they will send this for collection!

So two questions:

  1. Will they be stupid enough to send this for collection?
  2. What happens if they do send it for collection?