Everyone has their own way of delivery: some, like me, deliver each tier separate-some box 'em, other don't; some deliver all stacked using the center dowel method.
Whatever method you use, most of us use that non-skid material to keep cakes from sliding around the back of your vehicle. If using a car seat, I'd always suggest using some sort of 'wedge' to level out the seat. I roll up a towel (or use a wedge of foam rubber)then place a sheet of non-skid over it and the seat.
Another thing you may need to consider is the sun shining on your creation and melting/fading everything. You can cover the windows with a special light filtering film...or once I just pinned a white sheet from the back of the car to the top of the front seats! 
And make up a sign to put into your back window to explain why you are driving so slow!! 
"Caution: Wedding Cake Delivery"
try sugarshacks successful stacking DVD. its really helpful! i use a similar method to her. i use SPS 4" dowels to support my tiers, stack them on top of each other, then drive two sharpened wooden dowels through all the tiers. the PP made a good point about sunlight through the windows! it can really heat up your icing, so covering your windows with those little shades or tinting your windows is a good idea if you deliver a lot of cakes.
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