Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.

/**
 * Add a standard $ value surcharge to all transactions in cart / checkout
 */
add_action( 'woocommerce_cart_calculate_fees','wc_add_surcharge' ); 
function wc_add_surcharge() { 
global $woocommerce; 

if ( is_admin() && ! defined( 'DOING_AJAX' ) ) 
return;

$county = array('US');
// change the $fee to set the surcharge to a value to suit
$fee = 1.00;

if ( in_array( WC()->customer->get_shipping_country(), $county ) ) : 
    $woocommerce->cart->add_fee( 'Surcharge', $fee, true, 'standard' );  
endif;
}