Codeable #64026 - compatibble with WooCommerce 2.6.4 and 3.x

wc-bp.php

<?php

/**
 * The message sender ID (0 for current user).
 */
define('CA64026_SENDER_ID', 1);

/**
 * What username to use for guests.
 */
define('CA64026_NO_USERNAME', 'no_username');

/**
 * The categories for which to send messages.
 */
define('CA64026_CATEGORIES', serialize ( [ 123, 321 ] ) );

/**
 * Trigger sending messages when an order is completed.
 */
add_action( 'woocommerce_order_status_completed', 'ca64026_order_completed_cb' );

/**
 * Sends message for products author in order.
 *
 * @param  Integer $order_id The order id.
 */
function ca64026_order_completed_cb( $order_id ) {
	foreach ( ca64026_get_order_products( $order_id ) as $order_product ) {
		ca64026_send_message_for_order_product( $order_id, $order_product[ 'product_id' ]);
	}
}

/**
 * Get order items by order ID.
 *
 * @param  Integer $order_id The order ID.
 * @return Array             The order items.
 */
function ca64026_get_order_products( $order_id ) {
	$order = wc_get_order( $order_id );
	return $order ? $order->get_items() : [];
}

/**
 * Get product author.
 *
 * @param  Integer $product_id The product ID.
 * @return Integer             The product author ID.
 * @return WP_User|bool        The product autho user object, false on failure.
 */
function ca64026_get_product_author( $product_id ) {
	$product = get_post( $product_id );
	return $product ? get_userdata( $product->post_author ) : false;
}

/**
 * Get the user login by id.
 *
 * @param  Integer $id The user ID.
 * @return String|Bool The user name or false on error.
 */
function ca64026_get_user_login_by_id( $id ) {
	$user = get_user_by( 'id', $id );
	return $user ? $user->get('user_login') : false;
}

/**
 * Send a customized message with BuddyPress.
 *
 * The message is send to the product author.
 *
 * @param  Integer $order_id   The parent order.
 * @param  Integer $product_id The product ID. 
 * @return int|bool|WP_Error   ID of the message thread on success, false on failure.
 */
function ca64026_send_message_for_order_product( $order_id, $product_id ) {

	$order   = wc_get_order( $order_id );
	$product = wc_get_product( $product_id);

	/**
	 * Skip product if it's not in one of the categories in CA64026_CATEGORIES.
	 */
	$product_categories = wp_get_post_terms($product_id, 'product_cat', ["fields" => "ids"]);
	$skip               = ! array_intersect( unserialize( CA64026_CATEGORIES ), $product_categories ); 
	if ( $skip ) {
		return;
	}

	/**
	 * Get the buyer username, guests will have the username defined in the constant CA64026_NO_USERNAME.
	 */
	$buyer_username = ca64026_get_user_login_by_id( $order->get_user_id() );
	if ( ! $buyer_username ) {
		$buyer_username = CA64026_NO_USERNAME;
	}

	// We need <br /> tags, and then we need to replace them with \n, as expected by BuddyPress.
	$billing_address = str_replace('<br />', "\n", wp_kses( $order->get_formatted_billing_address(), [ 'br' => [] ] ) );

	// I'm using this weird concatenation to both the message and indentation in my IDE correct.  
	$template = ''
		. 'Congrats, you have received an order from "%1$s", order #%2$d.' . "\n"
		. "\n"
		. 'The billing address is:' . "\n"
		. '%3$s' . "\n"
		. "\n"
		. 'The product purchased was "%4$s".' . "\n"
	;


	$content = sprintf( $template, $buyer_username, $order_id, $billing_address, get_the_title( $product_id ) );

	$message_args = [
		'sender_id'  => ( defined( 'CA64026_SENDER_ID' ) && CA64026_SENDER_ID )
			? CA64026_SENDER_ID
			: bp_loggedin_user_id()
		,
		'recipients' => ca64026_get_product_author( $product_id )->get('ID'),
		'subject'    => sprintf( __('New Order #%s', 'ca64026'), $order_id) ,
		'content'    => nl2br( $content ),
	];

	return messages_new_message( $message_args );
}