AusPost AddOn for MarketPress

Hi All,
I looked at doing this last year, but alas clients and a baby got in the way.
I'm wanting to do an Auspost addon for marketpress, since I'm guessing a built in option is probably a fairway off.
This will be my first hooragh in creating a addon for a plugin that isn't a hack job.
Just wondering if there is anything I should know about how MarketPress handles shipping before I start.