hi there
it is easy to implement
1. edit extensions\default_per_item_shipping\config.xml
find
<item id="default_per_item_shipping_cost">
<type>input</type>
<default_value></default_value>
</item>
and insert after it
<item id="default_per_item_shipping_fee">
<type>input</type>
<default_value>0</default_value>
</item>
we add new parameter in extension config.
2. add translation for this parameter.
edit extensions\default_per_item_shipping\admin\language\english\default_per_item_shipping\default_per_item_shipping.xml
<definition>
<key>default_per_item_shipping_fee</key>
<value><![CDATA[Fee:]]></value>
</definition>
3. now we need to change shipping cost calculation
edit extensions\default_per_item_shipping\storefront\model\extension\default_per_item_shipping.php
find
$quote_data['default_per_item_shipping'] = array(
'id' => 'default_per_item_shipping.default_per_item_shipping',
'title' => $this->language->get('text_description'),
'cost' => $this->config->get('default_per_item_shipping_cost') * $this->cart->countProducts(),
'tax_class_id' => $this->config->get('default_per_item_shipping_tax_class_id'),
'text' => $this->currency->format($this->tax->calculate($this->config->get('default_per_item_shipping_cost') * $this->cart->countProducts(), $this->config->get('default_per_item_shipping_tax_class_id'), $this->config->get('config_tax')))
);
and change it to
$quote_data['default_per_item_shipping'] = array(
'id' => 'default_per_item_shipping.default_per_item_shipping',
'title' => $this->language->get('text_description'),
'cost' => $this->config->get('default_per_item_shipping_cost') * $this->cart->countProducts() + $this->config->get('default_per_item_shipping_fee') ,
'tax_class_id' => $this->config->get('default_per_item_shipping_tax_class_id'),
'text' => $this->currency->format($this->tax->calculate($this->config->get('default_per_item_shipping_cost') * $this->cart->countProducts() + $this->config->get('default_per_item_shipping_fee'), $this->config->get('default_per_item_shipping_tax_class_id'), $this->config->get('config_tax')))
);