http://www.amazon.com/gp/help/customer/display.html/?nodeId=200876660

"Add On Item"

So what exactly does this designation mean?

See, the way I read it, it would seem like unless I buy $x worth of merchandise, they simply will not send it to me until I do, at some point in the future, amass $x worth of merchandise into my cart.

Is that the story? If so, that's genetically-modified-cucumber-up-the-ass gay. If not, then what the hell does it mean?

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Power is always in the hands of the masses of men. What oppresses the masses is their own ignorance, their own short-sighted selfishness.
- Henry George

I think it's stuff that's too cheap/small to merit free shipping on its own.

Yup, that's basically it. As Kd said, too cheap to justify shipping on its own. Or that's what they say. I think the minimum amount you have to reach is $25

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