Shipping costs + PayPal fees. Remember that a lot of pack and ship places charge extra (little mom and pop type places, which can be a lot more convenient than standing in line at the post office). I know I have tried to sell stuff with cheap shipping and I ended up making almost nothing, in some cases actually LOSING money.
I shipped two 24" aquarium light strips with bulbs. I had to buy cling wrap, because I was out ($3) sent it UPS (almost $17) and had to pay the Paypal fees (~$1.50)....I sold them for $25 shipped. I also had to scavenge Styrofoam and a box from work. When you consider my gas to drive to Staples, I basically lost money, but the buyer was very pleased.
If you only charge the actual shipping cost, selling a small amount of plants is mostly a labor of love...