Adding 'extras' to a product

Hi there.
I'm setting up a software reselling outlet. The software I'm selling is security software and as such has extras available at various costs. I'd like to have something along the lines of the following:

• Main Product offered at a per-user cost, number of users selectable from quantity dropdown
• Optional add-on A with the same number of users as Main Product, selectable via tickbox
• Optional add-on B with the same number of users as Main Product, selectable via tickbox

and ideally

• Optional add-on C with the number users as selectable from a dropdown, no more than the Main Product

I understand the boundaries of quantity can be set, that's cool - I'm looking for some pointers to read up on how additional products can be set up on the same page as the main product. An example of what I mean (minus the C bit) is here:

http://www.bigpockets.co.uk/product.php?lang=&submenu=&menu=&product_id=22272

essentially, buy Main Product, have the options of adding A, B and C.

Hope this makes sense - thanks in advance for any pointers.

Pete

As attributes but use radio buttons so that only one can be selected as check boxes can all be checked

Perfect - thanks kobra.