I'm not doing Commerce (International) OR International Studies. And I'll be going on exchange for a year next year. I will not pay the tuition fees of the overseas university, but instead I pay UNSW my normal fees (as I would if I was still in UNSW, via HECS), as well as all my own living expenses and airfare while abroad.
Also, if I'm not wrong, in order to stay in the "international" stream, you will have to have a credit+ average. Which is what you need to apply for exchange as a student from any other degree anyways. And you have to add another year on top of the regular commerce degree to just basically "do something overseas for a year which adds an extra year to your degree and HECS debt" when you can "do something overseas for a year that would count towards your degree and let you graduate a year earlier" under just a normal Commerce degree... All that without adding on an additional year of HECS debt. Plus, the UAI is lower, lol, though I see the UAI isn't really your main concern.
Oh and with the Commerce (Liberal Studies), I use to think it would be fun doing all different kinds of subjects from several different faculties. Then I was told by someone who did that degree that it's not as flexible as they say. You cannot do more than a certain units of subjects from a certain faculty. And if you wanted to study about something in depth (other than business related subjects), you might be left unsatisfied as you will not be able to do more than just a few subjects. I say, do Commerce/Science instead if you're more interested in science/maths/psychology stuff and it's also 4 years (same length as liberal). Or if you're in for some other language or history/politics/art subjects as well as some science stuff (yes apparently you can co-major in something like mathematics in an arts degree...), do Commerce/Arts if you don't mind the extra year on top of the four years.
But at the end of the day, it's your choice.