I don't think the "12 plants and under" rule applies to shipping.
"Q. Do I need a permit to bring back plants from my travels abroad?
A. If you bring back 12 or fewer articles of admissible plants that have no special restrictions, you do not need a permit. However, you do need a phytosanitary certificate from the country of origin."
Source:
http://www.aphis.usda.gov/plant_heal...ing_faqs.shtml