Can NOT purchase additional sensor without purchasing entire Base unit

After adding additional sensor and FOB keys to the cart the system adds the base system and pad automatically to the cart, there is no way to delet them We do not want that, we already purchased the entire system a few days ago.

Two questions:
1. Why did the manual said you can purchase additional sensor but failed to inform the customer you must also purchase the base unit and keypad again???
2. Do we need an RMA# to return this product???