The question here becomes that of agency. An agent (herein physician) can bind a principle (you) to a contract.
Agency can be binding in three ways. They are:
EXPRESS AGENCY - the agent has actual authority from the principal. Example is the power of attorney from above.
IMPLIED AGENCY - the agent has implied authority from the principal. An example would be an director or officer of a company - while they are not expressly representing the company, it is implied they are, and anything they sign would be binding unto the company.
APPARENT AGENCY - that there was apparent authority. Example is if you walked into a store, and asked how much an item is. A person who worked there told you $40 and rang you up for $40. Later you find out that the item was actually $150, and the employee did not even have the right to ring you up. Regardless, they had apparent authority to you, and the price quoted by the employee with the apparent agency is binding upon the business.
As such, someone in your situation wants to send the company a letter, stating that the physician was not acting as one's agent, and had no express authority from you - but perhaps even more importantly - they should have known that he had no express or implied authority, either. As such, then someone in your situation is not bound by whatever agreement the physician orally induced or signed, and this includes their auto-supply clause.
One wants to send back the products not used and unpaid for with the letter. At this point, the company may either drop the matter, or, try to sue for breach of contract, in which case the patient would have to show the Court that the physician's actions did not amount to an agency establishment (thus binding him). Alternatively, the patient can even file the lawsuit one's self first - this is called a declaratory judgment - and ask the Court to determine whether or not an agency existed. Most however would simply send the letter and hope the company drops the matter.
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