When using multiple categories but you want a single flat rate shipping value for the whole shipment (rather than per category), you need to set this up using javascript. Currently, if all of your categories had a flat rate shipping value of 9, having a product from two different categories would equal $18 shipping rather than a flat $9.

Update all categories to 'Shipped using a flat rate fee' with a value of 0 in the 'Product Delivery Option' section.

Paste the following right before the closing </head> tag in your checkout template, and update the value to be whatever shipping cost you need:

<script type="text/javascript" charset="utf-8">
jQuery(document).ready(function() {
  FC.checkout.config.orderFlatRateShipping = 9;
  FC.checkout.updateShipping(-1);
});
</script>

This will make the shipping charges $9 no matter what is added to the cart.

• http://forum.foxycart.com/comments.php?DiscussionID=4389&page=1#Item_14