Australia post plugins for domestic and international shipping

I have rates that are discounted from Australia post which i cant get to be used in any plugin i can integrate these rates into a shopping cart and everything we ship the customer is over charged for the shipping we only want to charge the customer what we are paying. the rates are in an excel spread sheet where we can choose the product the customer zone weight etc - the difference can be 40% this is only being effected for the international deliveries the domestic is working.
very frustrated.