Errors with new custom shipping type

Basically, I created a new shipping type called 'Delivery' to go along with 'Small Package' by creating a small module with the following code:

function uc_custom_shipping_type(){
  $weight = variable_get('uc_quote_type_weight', array('customer_delivery' => 0));

  $types = array();
  $types['customer_delivery'] = array(
    'id' => 'customer_delivery',
    'title' => t('Delivery'),
    'weight' => $weight['customer_delivery'],
  );
  
  return $types;
}

So this creates for me a new shipping type called 'Delivery'

Now, I set Delivery to be the store default shipping type in Store > Configuration > Shipping Quote Settings.

I then created 2 Flatrate quotes 'Delivery quote' and 'Postage' and related them to our 2 types as follows using workflow-ng:

If order contains a product with shipping type 'Delivery' => Assign 'Delivery quote' quote
If order does not contain a product with shipping type 'Delivery' => Assign 'Postage' quote

This all works wonderfully well: as long as I explicitly mark 'Delivery' as 'Default product shipping type' in the product edit form, I get my 'Delivery quote' whenever the product is included, even if it is mixed up with other products.

However, if I leave it at the in the product edit form (which is set to delivery) then I get the 'Postage' quote. Debugging indicates the product indeed has the 'Delivery' shipping type, but somewhere in workflow or quote it is not getting picked up.

(ps: congratulations on a wonderful module. My client, who is by no means technical, is getting great joy from using it. In fact that's kind of the problem; his enthusiasm is such that he's gone and added 400+ products already, so I'd kind of like the defaults to hold :) )