Variants with additional cost

I’m having a problem configuring a setup I need within options / variants.

Some of my products have the option to buy additional ‘tags’. These tags are 35 cents each. Further, they have a minimum threshold of 24.

So, if I have a customer who buys a quantity of 20 and then checks the options for tags - I need that check box to calculate and charge .35 each for 20 tags.

This seemed to work in a previous version of cscart however I don’t have access to that cart anymore.

Here is an example from another site: [url][/url]