If the second prescription is for a higher strength/dosage, your insurance should cover it. For instance, if someone went to urgent care and received a prescription, and then saw their PCP, who changed the prescription, that would not be doctor shopping. Going to different doctors and then paying cash for a script when you have insurance is what throws up the red flag...any doctor (and law enforcement) can check the database and see exactly what was filled, when, where and how.
I don't understand the question. Cash is still legal tender in American pharmacies as far as I know.
However, if you are talking about defrauding your doctor through the practice of "doctor shopping", you are placing your access to opioid medication at risk in America's paranoid world of controlled drug monitoring systems, used by pharmacies in 49 of 50 states.