eCart stores the shipping state selected in a session called "eCartCheckoutForm_shipping_state_province"
you would need to hand code an array of all the states to the west of the mississippi:
$toTheWest = array();
$toTheWest = "CA";
$toTheWest = "WA";
$toTheWest = "UT";
$toTheWest = "TX";
then set a session if the selected shipping state is in the $toTheWest array:
$_SESSION['toTheWest'] = 0;
if(in_array($_SESSION['eCartCheckoutForm_shipping_state_province'], $toTheWest)) $_SESSION['toTheWest'] = 1;
in eCart create a new shipping rule as:
Based on Session Variable value:
if the session variable "toTheWest is "=" a value of "1"
Flat rate charge of "3"