It's more than likely you won't get an offer because Commerce has high participation rates and their enrolment quota is usually filled. Other degrees don't get full enrolment from those with adequate ATAR's for the cutoff, so they accept people below it (without special consideration). My case, for example, I was nearly 9 full points below the published cutoff for Bachelor of Engineering (Software) and still got offered it in the main round.
The thing you can do, however, is put Economics at the top of your new preferences (if your bonus points still apply), then a BA/BSc double, whilst doing courses relative to Commerce. You can then transfer over after first Semester (or after 1st year), given your WAM is high enough and do whatever you want to do in Commerce.