The short answer is "You don't".
The long answer is you will need to modify the ozpost.php code itself. This probably won't be too difficult because the the code calls a function (_get_cost_on_error) that, as the name suggests, gets the postage cost if there is an error.
All you need do is modify this function so that it returns the cost you desire. Currently all this function really does is retrieve the values from the database and determines whether to return the local value, the overseas value or whether to disable the module if the value is zero.
Hope this helps.
Cheers
Rod
ps. I quite like this idea, it seems more practical than the fixed costs.
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