gistfile1.php

add_filter( 'wpsc_update_customer_meta', 'wpsc_single_target_market', 15, 2 );
add_filter( 'wpsc_get_customer_meta'   , 'wpsc_single_target_market', 15, 2 );

function wpsc_single_target_market( $result, $key ) {

	if ( $key !== 'shipping_country' && $key !== 'shippingcountry' ) {
		return $result;
	}

	$countries = WPSC_Countries::get_countries_array();

	if ( count( $countries ) !== 1 ) {
		return $result;
	}

	$country = $countries[0];

	return $country['isocode'];
}