Further Insurance Stupidity

So I called the ins co, United Healthcare, today to ask what the missing piece of evidence was. They sent a request (I never recieved it) for proof of concurring insurance. I explained that the proof of no lapse in insurance and concurring insurance were explained in the same letter anyway! Proof of this, proof of that, all in hopes that patients will pay out of pocket rather than hassle with it, I'm sure.
"Managed Care"-as in managing to make their stockholders wealthy and their so-called customers poor and sick.
A few moments of silence and the phone person, who was pleasant enough, said she's resubmit the claim and that I didn't need to go through the appeals process like it stated in the letter.
So we'll see what happens next. All this over $84? Well, folks, I can't afford $84 especially when my husband puts out twice that per month to pay for so-called insurance. This only one of two disputed claims. Of course when I call the Drs office to find out if they've taken any action on their end, they cheerfully inform me that I can pay it and then wait until the ins co pays, if ever, for credit...and they never let on if they've tried billing the other insurance I had at the time.
How can I have two valid types of insurance and still be stuck with the bill?!