This type of shop sells weapons. Each time you discover a new weapon shop, an upgrade for each weapons in preceding shops is unlocked (up to a maximum level of 9). Each upgrade increases attack by 1.
The first weapon shop is located at 0.3 km and after that every 1.2 km.
By the 23.1 km Weapon Shop, all weapons will have been made available. Then, the weapon shops will start looping their wares.