First, the route I'm currently flying - started off at the galactic east and headed west, looks like I still have plenty of time to reach all of the destinations shown before I run into time problems, even if I have to stop for maintenance a couple of times:

Next, my current cargo and missions - remember that this is a Planet Express, a small lightly armed ship, with a lot of cargo space taken up by passenger accommodation:



I can't actually remember how much I've put down in deposits for the cargo, but I stand to make a stonking profit by the end, especially on parcels, which cost me nothing to carry. And this isn't an unusual flight; I'm being offered gems, platinum, and gold as cargo at most of the stations I stop at, also high-fee packages and passengers, but at this point mostly they're too far off the route I'm flying to be worth taking the risk.
Bottom line, I really think there should be some mechanism to ensure that you can't go on taking aboard high-value cargo and mail indefinitely. Maybe insurance premiums that pile up the more you carry, or taxes of some sort.