That all comes down to what is most important to those passengers/shippers: money or time. I was looking at international flights recently, and understand I'm firmly in the time category. While it would've been nice to save half the cost of the fare, doubling travel time and having 2 stops en-route was not within my acceptable parameters.
<Shrug> sure you could go SpacePriceline.com to fill those holds. I would argue that if that's a IMTU that most bargain shippers would consistently wait until the last minute to force a bargain, that there might be a 'bargain broker' that specializes in knowledge of empty holds and bank accounts to work out a going rate of below normal rate shipping.
Might also be a way to at least fill holds at cost from low pop worlds with otherwise low cargo generation. Get an extra die for each 10% off for instance.
Might also be standard practice that if you cut rate for some of your shippers, all will demand the rate. From now on.
Another direction I've gone is cutrate shipping, because the ships themselves are obsolete, unflagged, uncertifed, on the run from the bank, pirated, and if you are lucky not operated by cannibal pirates or leaky reactors.
No mortgage or investor service, no stinking rules.
No legal protection or recourse if something goes wrong.
They never see the 100D of any starport and any facility they see isn't better then E. Most likely it's an open space asteroid or discrete tertiary world handoff with one or more small craft operators with a talent or fix in for making cargo legit for final delivery.