String Theory Shoe for the isolation feet
The String Theory Shoe is for use with the String Theory feet. The foot locates into the shoe and it stops it sinking into soft surface like carpet. This would interfere with the free movement of the foot, reducing performance.
They are sold individually so you can just get as many as you need. They add 3mm to the height of the foot.
The shoes are not needed if the feet are to be used on a flat hard surface like a shelf of HiFi rack.