I've been waiting for this to be available for $20 for a long while. Finally got it. Fry's via Google Express has Planet Earth II 4K for $29.98 with Free Shipping. New Google Express customers receive a $10 credit for your first order. After creating a new account or using your GMail account with Express for the first time, during checkout there should be 2 Promos available, 20% off your first order or $10 off your first order. Use the $10 Promo to bring the price down to $19.98. Good luck.
Alternatively, Walmart via Google Express also has Planet Earth II 4K for a starting price of $31, but their free shipping doesn't kick in until $35.