A tonneau cover system is provided for a cargo box of a pickup truck. The tonneau cover system includes a framework system being positionable over the cargo box of the pickup truck. The system further includes a tonneau cover positioning system having a first positioner mount that is coupled or joined to the framework system. The first positioner mount has at least one downwardly-extending leg member extending from a main body portion to define a cross-sectional profile engageable with the cargo box of the pickup truck to inhibiting movement of the framework system relative to the cargo box of the pickup truck in vehicle fore and/or aft directions. In some embodiments, movement of the framework system can be inhibited in the vehicle fore and/or aft directions and also the cross vehicle direction. A bed rail extender clip is also provided.