that thing they want you to buy is a power wire with a fuse in it... u dont NEED it...
you DO need a 10 or 12 gauge wire, fed from the battery, with a 20 amp fuse within 12 inches of the battery, to protect the wire... if a non-fused 10 gauge wire shorts out, its gonna get really hot and probably start a fire in your car. Fuse the wire at the battery as well.