sure I can tell you how fastway work with labels. If you look at this page here it quite straight forwards.

http://www.fastway.com.au/RFRatesPDF/1/RS-MEL[Mar10]_web.swf

so each colour represent different destination. most of labels is quote correctly on ozpost because they one destination represent only one colour with 2 price which is frequent and non frequent as you understand.

the problem ozpost having is the local delivery because it has 4 different colours (blue, yellow, black, brown) (that's why fastway calculation page show them all together) and each colour represent how often the user use fastway in a month. hint the more you use the cheaper it get.

if you looks at melbourne for example if you use min of 200 each month then you only have to pay $4.02 each label, min 100 then $5.61 and so on...

Suggestion...
I think only way you can do this is you have to put in the price of each label manually, I can see this is not too difficult as in ozpost user can select what label they want to use.... therefore if user select blue then they should have brown, black, yellow turned off and when fastway return (blue/yellow/black/brown) you can then check which colour option user selected and use that price.

Now the hard part is that different local has different prices... so to make ozpost work perfectly, you probably have to input all the price for different location.. you can look number of location here http://www.fastway.com.au/1Prices.html
good news is black and brown for all the location is the same price.
and blue is always quote correctly because it's the price for non frequent user.
only problem is yellow which I think you can just average it and it should be really close enough for all ozpost customer to be happy :)

I will need to do abit more research with satchel and i will let you know.

Hope this help.