You received a letter from US Customs and Border Protection saying they had seized a shipment of a controlled substance that I absolutely did not order. Any advice?

Unable to answer - insufficient information is disclosed in the question. What ELSE did the letter say? If they didn't come knocking at your door with handcuffs I would be guided by whatever else MUST have been included in their communication. Remember, under the US system of justice you are innocent until proven guilty. Unless they have something else other than the fact that your address was on the package they are going to have trouble establishing probable cause.