Just as an example, that happened to me recently.
I got a bill from my son's pediatrician, it was for $49.63, I paid it through my HSA "send a check" feature.
The Dr.'s office lost the check and never deposited it.
So I called them up to see when they would cash it, they said they never got it.
I told them I'd like to pay it in full now, I got a 40% discount off the bill, for paying in full, right then.
Hospitals do this very same thing.
Basically, what these offices bill for, is not what they will actually take for cost of service + profit.
It's a sort of shell game, medical providers play.