The National Geographic Store has a free ground shipping promotion with no minimum purchase required! There is no coupon code to enter, the offer is factored in on their website. The promotion expires on 6/26/14 at 11:59pm pacific time.
There are some exceptions per their website: "Offer excludes framed maps, CafePress merchandise, Genographic Kits, Around the World in 125 Years - 3-Volume Collector's Edition Book from TASCHEN, mounted maps, and globes. U.S. standard delivery only. Additional charges apply for expedited and non-U.S. shipping. Cannot be combined with other offers; not valid on previous orders. Applicable for online orders only. Prices not valid in the National Geographic UK Online Store or Road Less Traveled Store."