http://smile.amazon.com/
The suggested charities were exactly what you'd expect. But the page allowed me to search for my own charity.
Entering "NRA" gave me a list including the NRA Freedom Action Foundation:
NRA Freedom Action Foundation
Location: Fairfax, VA | Year Founded: 2008
To educate Americans with respect to their individual rights as citizens, with particular emphasis on the Second Amendment to the Constitution of the United States.
Categories: Alliance/Advocacy Organizations
Programs: Nonpartisan voter research, education, outreach, and registration
Regions Served: Throughout the United States
Seems like a good place for my charity dollars to go.