fixed costs will be considered when inserting a customer (as described following the link above). Otherwise, fixed costs are not considered during insertion. However, they are considered in the objective function thus solutions tend to have less vehicles.
If you deal with a heterogenous fleet as described here: https://github.com/jsprit/jsprit/wiki/Heterogeneous-Fleet and you activate the fixed costs option like this: https://github.com/jsprit/jsprit/blob/master/jsprit-examples/input/algorithmConfig_considerFixedCosts.xml
fixed costs will be considered when inserting a customer (as described following the link above). Otherwise, fixed costs are not considered during insertion. However, they are considered in the objective function thus solutions tend to have less vehicles.