This is a clever way to get visitors to make a purchase by giving them a low-cost, but desirable item for free, they just pay the shipping. Example free items could be a product sample, t-shirts (which then advertise your business when worn) or a cool gadget. You can of course offer highly desirable but higher value items at prices people will know is a bargain as well.

This gets the money flowing and builds trust and allows you to market higher priced products to them via email using Purchase Events which can send them a specific email once this product is ordered and could offer a further coupon / preferential pricing or some type of special treatment that a regular customer doesn't get. They will also be on your email list if they have opted-in. 

Tip: Set your trip wire up for a limited period or as a limited offer which is stock controlled. This way you can run it for a short period to see whether it pulls in other sales and pays for itself. After all, the objective is to use this loss-leader to make money overall.