Cost Management Client Extension - Example of how to extend CSTACHKB for updating transaction cost

Hello,

We have requirement to update the cost to $0.00 for certain transactions like WIP Return for some specif items/scenario.

We are using FIFO.
In Cost management user guide, client extension section mentions that its doable through CSTACHKB.plb package.

In this package, function actual_cost_hook should return 1 to indicate hook has been used. Looking for what more to be done to mention/set transaction cost.

Wondering if there are any example of updating this package for updating/setting transaction cost.

Appreciate any pointers on this.

Thanks,

Neelesh Goyal