Option value product flat

I enter product options that need not calculate the amount, ie that is calculated only once for that product regardless of the quantity.
This, however, should not be a default function for all products, but an additional option to "+" and "-".

I have bought a couple of extensions that do this, but I have two other functions that work on options and create a conflict.
There is a good soul with a little 'patience I make life easier?

Have you tried this extension?
http://www.opencart.com/index.php?route ... on_id=2833