Peter,

I resolved the problem myself, Woothemes wasn’t very hulpful.
This needs to be added to QuickCSS to hide the blue (duplicate) order buttons in related products:

.related ul.products a.add_to_cart_button { display:none; } 
.related ul.products .avia_cart_buttons a.add_to_cart_button { display:block; }

And this is the modified wp-content/plugins/woocommerce/templates/loop/add-to-cart.php:

<?php 
/** 
* Loop Add to Cart 
* 
* @author WooThemes 
* @package WooCommerce/Templates 
* @version 2.1.0 
*/

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

global $product;

// ## DENIS - 31/07/2014 
$theurl = ($product->product_type == 'addons' ? $theurl = get_permalink( $product->id ) : $product->add_to_cart_url() );

echo apply_filters( 'woocommerce_loop_add_to_cart_link', 
sprintf( '<a href="%s" rel="nofollow" data-product_id="%s" data-product_sku="%s" class="button %s product_type_%s">%s</a>', 
// ## DENIS - 31/07/2014 
//esc_url( $product->add_to_cart_url() ), ## ORIGINAL 
esc_url( $theurl ),
esc_attr( $product->id ), 
esc_attr( $product->get_sku() ), 
$product->is_purchasable() && $product->is_in_stock() ? 'add_to_cart_button' : '', 
esc_attr( $product->product_type ), 
esc_html( $product->add_to_cart_text() ) 
), 
$product );

As you can see the Woothemes Products Add-on doesn’t take into account the “addons” product type when constructing the add to cart/view product button.
That’s all, and produces the desired and correct result.

Denis.