The official National Geographic Store has a pre-Thanksgiving promotion. Spend $100+ before then, and receive a $20 promotional code. You can find the complete terms and conditions over there, this promotion is heavily promoted on their website.
“Make a product purchase of $100 or more from the National Geographic Online Store from 10/1/15 to 11/19/15 at 11:59 p.m. PT, and receive an e-mailed promotion code the following day for $20 that may be redeemed in the National Geographic Online Store. Shipping and handling, taxes, and prior purchases do not apply toward the minimum $100 purchase requirement.”
PS: the free shipping promotion expired.
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