I might think the car experiment was missing one big item we deal with when shipping. Nobody threw the package! Part of the trauma of shipping is the sudden stops it may make along the way. That can often put the zip-lock out of order as it pops open. Auctions often prohibit zip locks for this reason as they tend to kill fish.
That leaves me thinking the package needs to be modified to suit the type and size of plant as well as how it is moving. If you are taking it to auction in your car, you can do a lot of things that won't survive a two day package trip. Part of that is keeping it moist so I like the idea of fully double bagged and sealed if it is a valuable group of plants. But then it also involves some discretion on what is added with the plants. Something wet will likely help to maintain the moisture on the odd occasion when the bag is split but then it can also act as something of a good or bad idea as well. Too much padding can become a battering ram if it moves too much. Just the right amount can be really needed to cut down the physical damage on other occasions.
The bottom line is there is no real way to guarantee the plants will arrive safe and sound. When you find a fish shipment setting on the porch and water has run off the edge, you may not want to look but with plants, you can still hope.