Pricing for GPL Organic Oats test OAT

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Getting frustrated. Got a bill from GPL for OAT test. When we had it done through previous doc, it costs $300. (Still does, I called.) We have a new doc, and now GPL is charging over $600. GPL claims it's the doctor's office that sets the price, but the doctor's office says that GPL sets the price! (Which makes sense, as the money will go to GPL...) I contacted GPL via e-mail and they want to know who the prescribing doc is ??? 

I'm getting very frustrated and won't pay this bill until I can find out what's going on! It seems to me that it wouldn't be very ethical or make any sense for doctor's offices to charge what they want, and the check end up with GPL anyway. Anyone else run into this problem? I feel like I'm missing something.