Is there something else included besides what the site states? Reason I ask is, the link you provided has the unit for $279.00??? Here's what it states on the link you provided:

Zt-2 the most affordable Wideband Air/Fuel Ratio Meter
Only $279 including the Wideband Oxygen Sensor!

http://www.zeitronix.com/Products/zt2/zt2.htm