Hi,
I think that you're not using the manual shipping methods plugin properly.
The manual shipping methods restrictions are global to the cart. When you set a max of 200g, it means that you don't want to display the shipping method when the total weight of the cart is bigger than 200g.
Which is why you can't add more than one product in the cart in that case.
Unfortunately, you cannot cumulate two different shipping methods. So even if you can have fixed shipping prices for these products using the "shipping price per product" option of the manual shipping methods plugin, if you have one of these products and australia post shipped products both in the cart, it will display both choices without cumulating both prices.
If you're ok with not being able to sell both types of products in the same order, then you can set big dimensions in these products (so that australia post doesn't show when these products are in the cart), create two manual shipping methods with a min volume of one of these products and the shipping price per product option turned on in them, and finally set the prices of the shipping for these products in the shipping price area of the product edition page.
Being able to have the two shipping methods system to cumulate would require to change the whole shipping system so that all the shipping plugins could be duplicated and that they would all have all the options of the manual shipping plugin. That's something we want to work on in 2013.