I'm not sure you covered this in the "purchase" portion. What if you buy legitimate books from Amazon's US store while over there. Did you make purchases that must be declared when you return? After all, you did buy stuff while out of the country.
I still remember returning from Europe (heavy duty hiking trip) after 3 weeks. Customs Lady read my form and said "You did not purchase anything?" NO, I replied. I get shipped to the detailed inspection line. Asked more forcefully "Are you sure you don't want to change your form and add purchases?" No, I replied.
She literally took everything out of one large suit case, two largish backpacks and went through all the items. She turns to me and says "You really did not purchase anything!" Off I went! Took more than half an hour after a Europe to Seattle trip - Pain in the butt!