public function getStates(): array
    {
        return [
            self::STATE_DRAFT           => 'Draft',
            self::STATE_OPEN            => 'Open',
            self::STATE_PAID_PARTIAL    => 'Partially Paid',
            self::STATE_PAID_PROCESSING => 'Paid (payments processing)',
            self::STATE_PAID            => 'Paid',
            self::STATE_WRITTEN_OFF     => 'Written Off',
            self::STATE_CANCELLED       => 'Cancelled',
        ];
    }