wc-virtual-order-complete.php

// Mark virtual orders as complete instead of processing
function rp4wp_virtual_order_payment_complete_order_status( $order_status, $order_id ) {
	$order = new WC_Order( $order_id );

	if ( 'processing' == $order_status && ( 'on-hold' == $order->status || 'pending' == $order->status || 'failed' == $order->status ) ) {

		$virtual_order = 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 ( ! $_product->is_virtual() ) {
						// once we've found one non-virtual product we know we're done, break out of the loop
						$virtual_order = false;
						break;
					} else {
						$virtual_order = true;
					}
				}
			}
		}

		// virtual order, mark as completed
		if ( $virtual_order ) {
			return 'completed';
		}
	}

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

add_filter( 'woocommerce_payment_complete_order_status', 'rp4wp_virtual_order_payment_complete_order_status', 10, 2 );