WooCommerce - Auto-Complete Orders. Skips "Processing" for instant "Completed" Order. For PayPal, AFTER IPN payment comes in to order, the payment is complete.
add_filter( 'woocommerce_payment_complete_order_status', 'wc_skip_processing' );
function wc_skip_processing( $status, $order_id ) {
return 'completed';
}