Flat option price when quantity increases?

Hello, I was unable to find a solution for the following important issue:

A product costs $10. You then have the option to customize it with a photo on it for an additional price of $5, no matter how many products are ordered (e.g. you can order 1 or 5 or 10000 items and the customization price will always be a flat $5).

Currently, when you buy, say, twenty items and choose the customization option, OpenCart will calculate it: (Product Price + Option Price)*Quantity = (10+5)*20 = $300 . Instead, what I want is something like (Product Price*Quantity) + Option Price = 10*20 + 5 = $205

Ideally, I would like certain options to ignore the quantity (so that it will be an additional $5 no matter how many products are ordered), while other option prices should go up with quantity (so that one product customization costs $5, two product customizations cost $10, etc.).

Surely, OpenCart must offer some way to deliver such a fundamental customer experience?

try this extension:
http://www.opencart.com/index.php?route ... 390&page=6

Thank you for your suggestion! Truth is, I really couldn't find an easy & free solution to the problem, so I eventually purchased - http://www.opencart.com/index.php?route ... on_id=1888 - it's got the Flat Fee feature (+ a bunch of others). I'd recommend it to anyone who's looking for a solution.