Automatically mark WooCommerce orders complete, only if all the products in the order is downloadable.

woocommerce-auto-complete-downloadable-orders.php

/**
 * Auto Complete all WooCommerce orders with only downloadable products.
 * If all the products are downloadable, the order status will be updated to 'completed'.
 * Use this plugin to implement the function:
 * https://github.com/woocommerce/theme-customisations 
 */
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );
function custom_woocommerce_auto_complete_order( $order_id ) { 
    if ( ! $order_id ) {
        return;
    }

    $order = wc_get_order( $order_id );
    $items = $order->get_items();

    // Check if all products in the order are downloadable
    $all_downloadable = true;
    foreach ( $items as $item ) {
        $product = $item->get_product();
        if ( ! $product->is_downloadable() ) {
            $all_downloadable = false;
            break;
        }
    }

    // If all products are downloadable, mark the order as completed
    if ( $all_downloadable ) {
        $order->update_status( 'completed' );
    }
}