How to add a fixed fee to a multiple items order?

Hi everyone. I need help. I would like to find a way to set a fixed price for small items that are purchased together.
Let me give you an example: A customer puts 12 items in the cart, I would like a fixed cost of €25 to be added to the checkout. The same cost should also apply if the customer buys only 8.
How can I do?
Negative discounts?

Hi @centunesima
To make it simple
You can create variants for multiple quantity items fixed price
Like create a product option Quantity and add values 8, 16 etc.. then you can assign fixed price to them.
But this way you won’t be able to track inventory. I mean, it is single product with just a 8 option not 8 quantity of the same product so if inventory is a problem for you then it is best suited to you
Thanks!

Thanks a lot for the tip. I set the items the way you sugested. Much appreciated.