Add-on for product - MarketPress

I cant seem to figure out where to put an add-on for a product.

For example, a person choses a quantity for a product, then checks a box to 'add on' something to that product that costs extra... or choses yes or no for the option.

I need to add a hosting product, and there needs to be a way for someone to select whether or not they want to add a cpanel license to it.

Let me know how to accomplish this..

Thanks!