Apple in-store configuration options (for a MacBook Air)
Apple Retail stores carry the four "stock" configurations of MacBook Air, as well as what they call the "ultimate" model. The "ultimate model" is the 2.0GHz i7, 512SSD and 8GB RAM.

The model you desire (1.8GHz i5, 256SSD, 8GB RAM) is not sold in stores. You would have to choose between the "ultimate" model and the base configuration (which has 4GB RAM).