Why is it so expensive to ship a single solar panel?

That is a really good question and one that is asked on a regular basis.    There are a number of reasons that it is expensive to ship a single panel. 

1. The biggest reason is that they need to go freight.  Most panels over 200W are too large to ship small package even through they weigh less than 150 lbs. 
2. Although the panels are sturdy, they need to be protected in shipping to reduce the changes that they will be damaged and the glass will shatter in transit.   In order to pack the panels correctly, they need to be wrapped on a wooden pallet and covered for protection. 
3. When the panels are loaded on the truck, they can't be stacked, so they take up a full column of room on the truck even when the pallet contains just one panel.

Here is a photo that shows how extensively panels need to be packaged once a secured pallet is broken from the manufacturer.  As you can see, the packaging needed is very extensive. 

In many cases companies can ship 10 panels for the same cost as shipping just one panel given the packaging, room required on a truck due to the inabiliity to stack a pallet of solar panels during shipping and the weight and cost of the packaging matterials.   When you factor in the high cost of gasoline charges from the freigh companies, it starts to make sense why shipping just one panel is so expensive.