I have actually come across this term while I was posted on a client's site. What "par la navette" meant in this particular case was that someone would drive around all the sites on which we worked and collect and distribute internal mail. Most of the documents were coming from or going to head office, but there was also some intra-site mail that was distributed by the "navette".

Given your brief sentence, I could imagine that these supporting documents are either handed over in person by the manager, or submitted via the internal mail system