The parts shop is located in the Driver Dome; it sells a variety of parts, items, and services. There are four tabs that separate different functions of the shop - "Purchase", "Modify", "Combine" and "Convert".
The first tab, the one shown by default, sells N parts as well as a few types of items. However, all of these are overpriced, especially the parts, which also give the lowest possible bonus.
This is where you can use Mittron Enhancers to add to the bonuses given by parts. It tells you the starting and finishing stats of the part and the cost to modify.
This tab allows you to use three parts of the same class and type - but not necessarily the same level - to create docking parts. It also gives you the ability to add a "seed part" - purchasable only by GCoins - to improve your chances of getting a good docking part.
This tab allows you to do a sort of equivalent of "Modify" to option parts, using the "Option Converting Solution"s that can be found at the store and in the Auction House.