1) How have you configured the USPS Shipping rate lookup on the confirm page? Is the rate lookup set to use ounces or Pounds? it's impossible to guess at thew cause of the problem whith out further details on how the rate lookup was configured, perhaps you could send shoots?
2) Clearing the cart does not clear the Session that is created by the USPS shipping rate look up.
Edit the eCart object. on the shipping tab, edit the USPS shipping rule, add a second trigger, Based on number of unique items in the cart > 0.
3) make sure that the weight being used for the rate lookup is set to use the TotalColumn of TotalWeight:
<?php echo WA_eCart_FormatNumber($<your Cart Name>->TotalColumn('TotalWeight'), true, 1); ?>
where <your Cart Name> is the name of your eCart object.
4) If you want it to say something besides grand total, you can certainly edit it to say whatever you like.
5) you would need to apply validation to the checkout page using the server validation from CSS Form builder to prevent the checkout form being submitted when blank.
