Lyman,
Another St Ivian is correct in what he says about STCs. But, an important fact is not addressed, that being liability. Take the case of modifying an aircraft with a devise that was not included as part of the original Type Certification by means of an STC. As holder and owner of the STC, you now assume liability for that devise, should it, for whatever reason, fail resulting in damage to the aircraft, personal injury or loss of life. The original aircraft manufacturer is absolved. In other words, STCs come with responsibilities in addition to ownership and or profit making opportunity. Just so you know...
TD