HUVs

I have a HUV 4421 DXL Diesel Ser # RK1221-284910 My question is about the Reverse Buzzer. My dealer states that it is a Federal mandate that these types of machines should all have them. When I contacted Husqvarna directly they stated it is an option and cannot tell me what it will cost to order and install myself. I have a very active 6 year old son and I do not want to even think about a possible back-up roll-over injury!!!