Pre and post commands count as part of the job operation and their success or failure should also be written to the job log - hence creating the job logs is the very last operation performed when a job runs. The post command is therefore run after the end of data processing within the job, but not after the end of all job operations
For you to do what you want, you will need to schedule your script for after the job completely finishes (which means it cannot be schueld as part of the job configuration and you will probably have to use task scheduler or possibly some external utility triggered by, perhaps, something like a job sucessful SNMP alert if you have such a capability.)
Note: There is actually a related problem with the pre commands as they have to start within the job as one of the job operations, which usually means you cannot run tape utility jobs as pre-commands as the main job will already have been allocated access and reserved the tape devices. resulting in the utility job being queued.