Autocomplete WooCommerce orders that contain only virtual products.

functions.php

<?php

/**
 * Autocomplete orders that contain only virtual products.
 * If the order contains a non-virtual product the order will not be completed by this function.
 *
 * @author  Mike Hemberger @JiveDig
 *
 * @version 1.0.0
 *
 * @return  string  Order status.
 */
add_filter( 'woocommerce_payment_complete_order_status', 'prefix_autocomplete_virtual_orders', 10, 2 );
function prefix_autocomplete_virtual_orders( $order_status, $order_id ) {

	$order = wc_get_order( $order_id );

	if ( 'processing' != $order_status ) {
		return $order_status;
	}

	$all_virtual_products = false;

	if ( count( $order->get_items() ) > 0 ) {

		foreach( $order->get_items() as $item ) {

			if ( 'line_item' == $item['type'] ) {

				$_product = $order->get_product_from_item( $item );

				// If not a virtual product.
				if ( ! $_product->is_virtual() ) {
					$has_non_virtual_product = true;
					break;
				}
			}
		}
	}

	// If no non-virtual products.
	if ( ! $has_non_virtual_product ) {
		$all_virtual_products = true;
	}

	// All virtual products, mark as completed.
	if ( $all_virtual_products ) {
		return 'completed';
	}

	// Non-virtual order, return original status.
	return $order_status;
}