1) maybe a better tactic would be to use the Set Session Value behavior on the cart page to set a session value for each column, then use the session variables in the Checkout Wizard. to add a set session value behavior, open the server behaviors panel, click the lightning bolt and select eCart -> General -> Set Session Value.
2) Sorry, was mistaken in my original reply, I thought you where asking about whith measument, not length.
the Australian Post API does not support other length measurements, you must pass the length in MM.
3) Add the folloing code at line 1 of the cart display page:
<?php if(isset($_SESSION["yourCart_AusPost_Quote"])) unset($_SESSION["yourCart_AusPost_Quote"]); ?>
where "yourCart" is the name of your eCart object.