Automatically complete woocommerce orders when paid

functions.php

// Automatically complete woocommerce orders on successful payment (skip "processing") - Robin Scott for SiliconDales.com
add_action( 'woocommerce_thankyou', 'sd_woocommerce_auto_complete_order' );
function sd_woocommerce_auto_complete_order( $order_id ) { 
    if ( ! $order_id ) {
        return;
    }
    $order = wc_get_order( $order_id );
    $order->update_status( 'completed' );
}