I'm not sure why you aren't able to make the modification through the interface, but you can manually add the multiply by 1.15 by opening up your WA_eCart/[storename]_PHP.php file and doing a search on the name of the rule without spaces. So if your shipping rule is called USPS Shipping, search for USPSShipping. Then you would add * 1.15 to the $totalShipping variable like this:
//eCart Rule
function WAEC_store1_USPSShipping() {
$totalShipping = 0;
if (true && (("USPS"=="USPS")&&(isset($_SESSION["store1_USPS_Quote"]))&&(floatval($_SESSION["store1_USPS_Quote"]) != 0))) {
$totalShipping += floatval($_SESSION["store1_USPS_Quote"]) * 1.15;//Result
}
return WA_eCart_FormatNumber($totalShipping, $this->ForceDecimalsC, $this->DecimalPlacesC);
}
//eCart Rule
the part in bold is what you want to add.