We had a lovely little night-freight airline in the 80's using BE20 and BE90 . As PH-SCP says you can trade fuel for load and 1100kg on the BE90 for an hours flight or 1400 kg load on the BE20 for 2/3 hours was , as I remember , OK.
Small parcels and newspapers are easily loaded into the standard version and we had one that did night-freight and then converted to pax fit and did a scheduled service for Sabena, an hour later.
Try to get in with the parcel carriers at their hub.
Of, course , in those days , it was a single pilot operation - night freight in Europe. We had funerals published on the roster ( not too far ahead ) and the CAA eventually changed the rules.
If you need a Captain - PM me !