In terms of maths, there are plenty of bridging courses. Usyd also has a "learning centre". which the econometrics teachers keep asking people to go to if they are having any problems with the maths aspects.
A knowledge of calculus is definitely helpful, but nothing has really been formally tested, and if you didn't understand calculus you could probably still follow most things - its statistics. I get confused in the logic and application of what we learn, rather than the underlyng mathematical concepts.
Economics has had some mathematical factors, but they've been an adjunct to the course rather than a core component, i.e the lecturer would say "for those of you who understand maths, this can be proven by...." but it would not be tested. A lot of people are doing eco as part of an arts degree, so it really depends what aspects of "commerce" interest you enough to choose the double degree.
As was previously mentioned, economics as a social science is becoming really popular as a degree as well. Its basically a degree that allows you to do things like economics, world history, political economy, a language even as well as any more commercey units (i.e metrics etc..) which may appeal.
Anyway, good luck with the decision - you still have heaps of time, so research as much as you can
