Plates and pillars, no dowels!! I swear by using plates with pillars that are trimmed to the height of the cake tier with PVC cutters! My plates have a 1 1/2" circle drilled into the centers for allowing the center dowel to go through all the tiers. Each cake sits on its own "table."
I worked for a caterer and we did up to 45 wedding cakes a weekend.
Needless to say, we did not set up or deliver cakes ourselves. They were sent with the catering servers to the events. Read: no cake experience whatsoever, except for maybe eating cake!!
I can't tell you how many times we had to salvage a cake or redo one entirely due to delivery error! But we never had one collapse! Usually it was the way it was put into the delivery vehicle...like next to stacked cases of liquor bottles that fall onto the cake when they slam on the brakes. A couple were dropped. A table gave way.
If you are going to let your customers drive your cakes home, try the plates and pillars. There are some very inexpensive brands. I just add them to the price of the cake, and the client appreciates not having to return stuff. Bonus cupcakes if they do.