Knobs, sequential plates, springs and mounting kits for the shifter in your build.
A heavier or differently shaped knob changes how a shift feels, and conversion plates can turn an H-pattern shifter into a sequential one.
These parts are made for specific shifters, so this page shows accessories from the same brand as the shifter in your build.
Most shifter accessories cost €15–€80. Conversion plates that switch a shifter between H-pattern and sequential modes are the priciest common upgrade, since they involve more than one part.