Fixed price for option regardless of quantity

I would like to add a fixed fee to a product that is independent of the quantity ordered.
For example, a customer orders 200 items at 0.25 each. They can then choose a fixed price option for that item, at a cost of 5.00
So the total price would be 200*0.25 + 5.00 (if the third option is chosen).

1. Is this possible using Hikashop configuration? or
2. By modifying a pricing function, perhaps along with a custom field?

Hi there,

Hikashop doesn't allow you to add fixed extra charge on a product.
One option is to use the manual shipping, activate the "use price by product" option and then, in the edition of your product, add a 5€ charge on the product.
By this way, customers will be charger 5€ more when they buy this product but not the others

Thanks Eliot, I was afraid you would say that!
I was thinking that I might add a custom field to indicate that a product could only ever have a quantity of 1. I would then need to change the code where it updates the price to interrogate that field and override the quantity if it was true.
Do you think that is feasible? If so, what module would need to be changed?

Thank you Nicolas - just what I need!

How do I need to change the error message? Where is it possible? Thnx.

At this moment I can't continue the order proces in frontend caused by the error message. How can I solve this problem?