From the Super Mario Wiki
The Mail Truck is an obstacle featured in Mario Kart: Double Dash!!. It is found in only two courses: in Mushroom Bridge of the Flower Cup, and in Mushroom City of the Star Cup. They are found driving only on the main streets and not on shortcuts, making it safer to use the shortcuts. The effect of hitting a Mail Truck has the same effect as hitting a banana, simply leading to a wipe-out.
The Mail Truck usually causes only one wipe out, but if caught in its front, it can cause up to three wipe outs.