Need some correction

I am plugin developer I am using your plugin thank you for your best product.. I Have created plugin for buddyboss or buddypress custom notification your plugin have conflict
with sending notification

Let me explain you

file location wc4bp\class\wc4bp-notifications.php (line number 28)
Here the function

add_filter( 'bp_notifications_get_notifications_for_user', 'wc4bp_format_purchased_notifications', 10, 5 );
function wc4bp_format_purchased_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {
    if ( 'send_purchase_notification' === $action ) {
		if( ! empty( $item_id ) ){
          $order         = wc_get_order( $item_id );
			$names         = array();
			$message       = __( 'The user %s has bought %s','wc4bp' );
			foreach( $order->get_items() as $item_id => $item ){
				$names[]   = '<a href="' . $item->get_product()->get_permalink() . '">' . $item->get_product()->get_name() . '</a>';
				$user_link = bp_core_get_userlink( $order->get_customer_id() );
			}
			$notification  = sprintf( $message,$user_link, implode( ', ',$names ) );
		}
        
    }
    
	return $notification;
}

observation : debug undefine notification variable when notification not found you need to return under the your action if condition another main point when I fired same filter so anther filter not working because you haven’t return $action after end function I modify your function below

add_filter( 'bp_notifications_get_notifications_for_user', 'wc4bp_format_purchased_notifications', 10, 5 );
function wc4bp_format_purchased_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {
    if ( 'send_purchase_notification' === $action ) {
		if( ! empty( $item_id ) ){
          $order         = wc_get_order( $item_id );
			$names         = array();
			$message       = __( 'The user %s has bought %s','wc4bp' );
			foreach( $order->get_items() as $item_id => $item ){
				$names[]   = '<a href="' . $item->get_product()->get_permalink() . '">' . $item->get_product()->get_name() . '</a>';
				$user_link = bp_core_get_userlink( $order->get_customer_id() );
			}
			$notification  = sprintf( $message,$user_link, implode( ', ',$names ) );
		}
        return $notification;
    }
    return $action;
	
}

Please make these correction on your next update please here you can check buddyboss-plateform plugin also followed same instruction

function bbp_format_buddypress_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {

	// New reply notifications
	if ( 'bbp_new_reply' === $action ) {
		$topic_id    = bbp_get_reply_topic_id( $item_id );
		$topic_title = bbp_get_topic_title( $topic_id );
		$topic_link  = wp_nonce_url(
			add_query_arg(
				array(
					'action'   => 'bbp_mark_read',
					'topic_id' => $topic_id,
					'reply_id' => $item_id,
				),
				bbp_get_reply_url( $item_id )
			),
			'bbp_mark_topic_' . $topic_id
		);
		$title_attr  = __( 'Discussion Replies', 'buddyboss' );

		if ( (int) $total_items > 1 ) {
			$text   = sprintf( __( 'You have %d new replies', 'buddyboss' ), (int) $total_items );
			$filter = 'bbp_multiple_new_subscription_notification';
		} else {
			if ( ! empty( $secondary_item_id ) ) {
				$text = sprintf( __( 'You have %d new reply to %2$s from %3$s', 'buddyboss' ), (int) $total_items, $topic_title, bp_core_get_user_displayname( $secondary_item_id ) );
			} else {
				$text = sprintf( __( 'You have %1$d new reply to %2$s', 'buddyboss' ), (int) $total_items, $topic_title );
			}
			$filter = 'bbp_single_new_subscription_notification';
		}

		// WordPress Toolbar
		if ( 'string' === $format ) {
			$return = apply_filters( $filter, '<a href="' . esc_url( $topic_link ) . '" title="' . esc_attr( $title_attr ) . '">' . esc_html( $text ) . '</a>', (int) $total_items, $text, $topic_link );

			// Deprecated BuddyBar
		} else {
			$return = apply_filters(
				$filter,
				array(
					'text' => $text,
					'link' => $topic_link,
				),
				$topic_link,
				(int) $total_items,
				$text,
				$topic_title
			);
		}

		/**
		 * @todo add title/description
		 *
		 * @since BuddyBoss 1.0.0
		 */
		do_action( 'bbp_format_buddypress_notifications', $action, $item_id, $secondary_item_id, $total_items );

		return $return;
	}

	if ( 'bbp_new_at_mention' === $action ) {
		$topic_id    = bbp_get_reply_topic_id( $item_id );

		if ( empty( $topic_id ) ) {
			$topic_id = $item_id;
		}

		$topic_title = bbp_get_topic_title( $topic_id );
		$topic_link  = wp_nonce_url(
			add_query_arg(
				array(
					'action'   => 'bbp_mark_read',
					'topic_id' => $topic_id,
					'reply_id' => $item_id,
				),
				bbp_get_reply_url( $item_id )
			),
			'bbp_mark_topic_' . $topic_id
		);
		$title_attr  = __( 'Discussion Mentions', 'buddyboss' );

		if ( (int) $total_items > 1 ) {
			$text   = sprintf( __( 'You have %d new mentions', 'buddyboss' ), (int) $total_items );
			$filter = 'bbp_multiple_new_subscription_notification';
		} else {
			if ( ! empty( $secondary_item_id ) ) {
				$text = sprintf( __( '%3$s mentioned you in %2$s', 'buddyboss' ), (int) $total_items, $topic_title, bp_core_get_user_displayname( $secondary_item_id ) );
			} else {
				$text = sprintf( __( 'You have %1$d new mention to %2$s', 'buddyboss' ), (int) $total_items, $topic_title );
			}
			$filter = 'bbp_single_new_subscription_notification';
		}

		// WordPress Toolbar
		if ( 'string' === $format ) {
			$return = apply_filters( $filter, '<a href="' . esc_url( $topic_link ) . '" title="' . esc_attr( $title_attr ) . '">' . esc_html( $text ) . '</a>', (int) $total_items, $text, $topic_link );

			// Deprecated BuddyBar
		} else {
			$return = apply_filters(
				$filter,
				array(
					'text' => $text,
					'link' => $topic_link,
				),
				$topic_link,
				(int) $total_items,
				$text,
				$topic_title
			);
		}

		/**
		 * @todo add title/description
		 *
		 * @since BuddyBoss 1.0.0
		 */
		do_action( 'bbp_format_buddypress_notifications', $action, $item_id, $secondary_item_id, $total_items );

		return $return;
	}

	return $action;
}
add_filter( 'bp_notifications_get_notifications_for_user', 'bbp_format_buddypress_notifications', 10, 5 )