public function get_coupon_codes() {
		$coupon_codes = array();
		$coupons      = $this->get_items( 'coupon' );

		if ( $coupons ) {
			foreach ( $coupons as $coupon ) {
				$coupon_codes[] = $coupon->get_code();
			}
		}
		return $coupon_codes;
	}