add extra fee - form in BO/order details - for customer returns ( order slip pdf)

[BoKr]

our online shop is based on Prestashop 1.6.14. We offer a return service for our customers. We charge an extra fee for this service of 4 €.

How can I implement this into Prestashop?

I am no coder until now but willing to learn. I assume it would be good practise to add a new field into the DB for this fee. Then a form field with the - editable- fee in BO/orders/order details at the bottom. So we can edit or activate this extra fee if used by customer when the customer returns products. The goal is that this field/extra fee appears on the order slip (PDF) and is properly respected in the accounting / bookkeeping functions of Prestashop. Ehm^^ Okay, most important is that we and our customers have the proper order slip for the tax authorities.

Thank you

 

[BoKr]

update

GOAL:

To have in the BO order details the oportunity to charge certain customers an extra fee!

We offer a special RMA service that is charged with 4 € at the moment. So, if a customer returns products AND! uses our service then I need the possiblity to charge him for this. The extra fee has to appear on the credit slip.

My idea:
1. add a form or optional button to order details in BO (best to hook it).
2. form should offer possibilty to take extra fee or optional button sends fix charge of e.g. 4€ to db.
3. clone ps_orders ID to extrafee table and link it with new extra fee ID.
4. edit credit slip tpl like: if $var_extrafee (may be combination of orderID&extrafeeID or so) then substract extra fee of 4€ from total