Build an add-on

cd wp-content/plugins
composer create-project paylexer/wordpress-starter paylexer-for-<plugin-slug>

wp package install paylexer/wp-cli-paylexer
wp paylexer scaffold paylexer-for-<plugin-slug> --platform=<plugin-slug> --platform-name="<Plugin Name>" --activate

 * Requires Plugins: <plugin-slug>

add_action( 'plugins_loaded', function () {
	if ( function_exists( 'paylexer_register_adapter' ) ) {
		return; // PayLexer 2.0.7+ is active.
	}
	add_action( 'admin_notices', function () {
		if ( ! current_user_can( 'activate_plugins' ) ) {
			return;
		}
		$installed = file_exists( WP_PLUGIN_DIR . '/paylexer/paylexer.php' );
		$url       = $installed
			? wp_nonce_url( admin_url( 'plugins.php?action=activate&plugin=paylexer/paylexer.php' ), 'activate-plugin_paylexer/paylexer.php' )
			: admin_url( 'plugin-install.php?s=paylexer&tab=search&type=term' );
		printf(
			'<div class="notice notice-error"><p>%s <a href="%s">%s</a></p></div>',
			esc_html__( 'PayLexer for <Plugin Name> needs the PayLexer plugin (version 2.0.7 or later) installed and activated.', 'paylexer-for-<plugin-slug>' ),
			esc_url( $url ),
			$installed ? esc_html__( 'Activate PayLexer', 'paylexer-for-<plugin-slug>' ) : esc_html__( 'Install PayLexer', 'paylexer-for-<plugin-slug>' )
		);
	} );
} );

protected string $id          = '<plugin-slug>';
protected string $name        = '<Plugin Name>';
protected string $by          = 'PayLexer for <Plugin Name>';
protected string $plugin      = '<plugin-slug>/<plugin-slug>.php';
protected string $plugin_url  = 'https://wordpress.org/plugins/<plugin-slug>/';   // any other URL (a paid plugin's pricing page) shows a "Get" link
protected string $description = 'Payments in <Plugin Name>.';
protected array  $supports    = array( 'payments' );

// Replace with the integrated plugin's own filter; find its name in that plugin's code or docs.
add_filter( '<plugin_slug>_register_payment_gateways', function ( array $gateways ): array {
	if ( ! function_exists( 'paylexer_register_adapter' ) || ! paylexer_is_connected() ) {
		return $gateways;
	}
	require_once __DIR__ . '/includes/class-gateway.php';
	foreach ( paylexer_get_gateways( array( 'test_mode' => true ) ) as $gateway ) {
		// "Dodo Payments", "Stripe", …, each extends the plugin's gateway base class.
		$gateways[ 'paylexer_' . $gateway['code'] ] = new PayLexer_<Plugin>_Gateway( $gateway['code'], $gateway['label'] );
	}
	return $gateways;
} );

use PayLexer\Integration\CheckoutRequest;
use PayLexer\Integration\Customer;
use PayLexer\Integration\Exception as PayLexerException;

// Called by the integrated plugin when the shopper pays with this gateway.
public function process( $record_id, $total, $currency, $email, $first_name ) {
	try {
		$checkout = PayLexer_<Plugin>_Adapter::instance()->create_checkout( new CheckoutRequest(
			gateway: $this->code,                                // this gateway's PayLexer code
			amount: (float) $total,                              // computed by the plugin, never typed by the shopper
			currency: $currency,
			customer: new Customer( $email, $first_name ),
			return_url: wp_get_referer() ?: home_url( '/' ),   // wp_get_referer() only returns allowed hosts
			reference: "<plugin-slug>-{$record_id}",
			order_url: admin_url( "post.php?post={$record_id}&action=edit" ),   // the dashboard links the Purchase ID here
		) );
	} catch ( PayLexerException $e ) {
		return new WP_Error( 'paylexer', $e->getMessage() );   // show it the plugin's way
	}

	update_post_meta( $record_id, '_paylexer_order_reference', $checkout['order_reference'] );
	return $checkout['checkout_url'];   // redirect the shopper here, the plugin's way
}

public function on_event( WebhookEvent $event ): void {
	$statuses = array( 'payment.completed' => 'paid', 'payment.failed' => 'failed', 'payment.cancelled' => 'cancelled', 'payment.refunded' => 'refunded' );
	$ignore_after = array( 'paid' => array( 'failed', 'cancelled' ), 'refunded' => array( 'paid', 'failed', 'cancelled' ) );
	$records = get_posts( array(
		'post_type'  => '<record_post_type>', 'post_status' => 'any', 'fields' => 'ids', 'numberposts' => 1,
		'meta_key'   => '_paylexer_order_reference', 'meta_value' => $event->order_reference,
	) );
	$status = $statuses[ $event->type ] ?? null;
	if ( ! $records || ! $status ) {
		return;
	}
	$current = (string) get_post_meta( $records[0], '_paylexer_status', true );
	if ( ! in_array( $status, $ignore_after[ $current ] ?? array(), true ) ) {
		update_post_meta( $records[0], '_paylexer_status', $status );
	}
}

// Replace with the integrated plugin's own refund hook; find its name in that plugin's code or docs.
add_action( '<plugin_slug>_order_refunded', function ( $record_id, $amount ) {
	try {
		$reference = (string) get_post_meta( $record_id, '_paylexer_order_reference', true );
		PayLexer_<Plugin>_Adapter::instance()->refund( $reference, $amount, 'Refunded in <Plugin Name>' );
	} catch ( PayLexer\Integration\Exception $e ) {
		// Show it the plugin's way: PayLexer refused the refund.
	}
}, 10, 2 );

add_action( 'add_meta_boxes_<record_post_type>', function ( WP_Post $post ) {
	$reference = (string) get_post_meta( $post->ID, '_paylexer_order_reference', true );
	if ( $reference ) {
		add_meta_box( 'paylexer-details', 'PayLexer Details', fn() => PayLexer_<Plugin>_Adapter::instance()->render_order_details( $reference, false ), '<record_post_type>', 'side' );
	}
} );