How to add a fixed price/una tantum in a dropdown

[joomlaccount]

Hi everyone, i'm stuck with this problem.
I need to add a fixed cost not related with the quantity of the product. So i have a dropdown choice with a "yes/no" option and when "yes" is selected i need a  final price (calculated by cart variant and quantity) plus fixed cost (this is the same rather the quantity is 1 or more).
I searched the forum and tried different things, but no results. I'm quite new to VM.

How can i do it?

Thanks

[John2400]

http://extensions.virtuemart.net/products/quantity-related-price-calculation-detail


Start here ..

[joomlaccount]

I check this out, but it doesn't seem what i'm for looking for. Any other suggestions?!