The easiest, most braindead cart system is google checkout. If you can stomach their 3% + 0.30 fee due to forcing you to use their particular payment processor, building a functional cart takes roughly 30 seconds. On the super-low end, their cart is fed its inventory through a Google Docs Speadsheet, which, while it might seem awful, is actually pretty efficient as far as letting you track your inventory. They give you options to either generate some html to put on your site for an integrated checkout that only goes to Google to do the final payment or to send the cart to google checkout. The system has promo coupons, etc, and for someone who doesn't need all of the fancy features of a full-blown cart, it's fine.
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