Receipt or invoice in the block checkout (Απόδειξη / Τιμολόγιο)

<?php

// Labels are in Greek; change them if you like.
$document_label = 'Παραστατικό';
$choices        = array(
	'receipt' => 'Απόδειξη',
	'invoice' => 'Τιμολόγιο',
);
$fields         = array(
	'company'    => 'Επωνυμία εταιρείας',
	'vat'        => 'ΑΦΜ',
	'tax-office' => 'ΔΟΥ',
	'activity'   => 'Δραστηριότητα',
);
$vat_error      = 'Ο ΑΦΜ δεν είναι έγκυρος.';

add_action(
	'woocommerce_init',
	static function () use ( $document_label, $choices, $fields, $vat_error ) {
		if ( ! function_exists( 'woocommerce_register_additional_checkout_field' ) ) {
			return;
		}
		woocommerce_register_additional_checkout_field(
			array(
				'id'       => 'snipfire/document',
				'label'    => $document_label,
				'location' => 'order',
				'type'     => 'select',
				'required' => true,
				'options'  => array_map(
					static fn( $value, $label ) => array(
						'value' => $value,
						'label' => $label,
					),
					array_keys( $choices ),
					$choices
				),
			)
		);

		// The invoice fields show, and are required, only when "invoice" is picked.
		$not_invoice = array(
			'checkout' => array(
				'properties' => array(
					'additional_fields' => array(
						'properties' => array(
							'snipfire/document' => array( 'not' => array( 'const' => 'invoice' ) ),
						),
					),
				),
			),
		);
		foreach ( $fields as $key => $label ) {
			$field = array(
				'id'       => 'snipfire/invoice-' . $key,
				'label'    => $label,
				'location' => 'order',
				'type'     => 'text',
				'required' => true,
				'hidden'   => $not_invoice,
			);
			if ( 'vat' === $key ) {
				// A Greek VAT number: 9 digits with a check digit ("EL" and spaces are fine).
				$clean                      = static fn( $value ) => (string) preg_replace( '/^(EL|GR)/i', '', (string) preg_replace( '/[\s.\-]/', '', (string) $value ) );
				$field['sanitize_callback'] = $clean;
				$field['validate_callback'] = static function ( $value ) use ( $vat_error, $clean ) {
					$vat = $clean( $value );
					if ( '' === $vat ) {
						return true;
					}
					$sum = 0;
					for ( $i = 0; $i < 8 && preg_match( '/^\d{9}$/', $vat ); $i++ ) {
						$sum += (int) $vat[ $i ] * ( 2 ** ( 8 - $i ) );
					}
					if ( ! preg_match( '/^\d{9}$/', $vat ) || '000000000' === $vat || ( $sum % 11 ) % 10 !== (int) $vat[8] ) {
						return new WP_Error( 'snippet_gr_invalid_vat', $vat_error );
					}
					return true;
				};
			}
			woocommerce_register_additional_checkout_field( $field );
		}
	}
);