How to add a price for COD payment method

What is the best way to do this?

My shipping provider (UPS, but others do the same) asks me a fixed rate for COD (cash payment at delivery), like 5€, and I want the customer to pay for it (and of course know it before he completes checkout).

I can't change the Shipping method because I am using UPS to automatically generate the shipping cost based on delivery address. I can't add a fixed cost to that unknown amount (I could add a cost in UPS settings but that would apply to any order unconditionally of payment method or anything else).

I can't use a tax because this is not a tax and taxes are in percentages, while I need to add a fixed cost (5€).

Could I add a line item (like it was a product)? How to do that? I cannot find a way to add a product to the cart in Rules.

I think this feature is VERY important for any physical ecommerce.

Thanks for your support.

Lorenzo