Most passenger airplanes also takes cargo containers (of a size specific to a group of plane types) below the passengers. Not that many of course, compared to a cargo plane. E.g. even a A321 takes 10 LD3-45 containers.
Depending on the density of what's being transported the lower volume might or might not matter. E.g. the a321, according to Wikipedia, has a max payload 25t (I assume with reduced range). That's not that hard to fill with 10 containers of 3.7m3 each.
I'd assume that one significant reason the flights are that cheap is that they have contractual cargo obligations requiring them to fly some flights anyway. A handful of additional passengers won't reduce the payload capacity meaningfully, but will still bring in more than the increased fuel/service costs.
Depending on the density of what's being transported the lower volume might or might not matter. E.g. the a321, according to Wikipedia, has a max payload 25t (I assume with reduced range). That's not that hard to fill with 10 containers of 3.7m3 each.
I'd assume that one significant reason the flights are that cheap is that they have contractual cargo obligations requiring them to fly some flights anyway. A handful of additional passengers won't reduce the payload capacity meaningfully, but will still bring in more than the increased fuel/service costs.