post.jar has hardcoded assumptions about what URL you want to hit and how it should behave – if you want to change those assumptions there are documented params for changing it. -Durl=... and -Dcommit=false.
if you want to post to something that isn't the XmlRequestHandler, you should specify -Dcommit=false, and then you can follow that with an explicit execution to commit...
java -Durl=... -jar post.jar *.cssv
java -jar post.jar
part of the reason it works the way it does now is that when commit=true it POSTs a single commit at the end of multiple file POSTs, if we use the param based commit it would either need to specify commit on all of them, or keep track of the last one only add the param there.
i don't object to changing post.jar to use a commit request param instead of sending the XML form, but this isn't a bug – it's working as it was intended.