add cost plus attribute/link attributes to options

My item is a ticket on a weekly bus trip (http://www.gtaskibus.com/). We have two packages in the store (http://shop.gtaskibus.com) the basic package with is just the bus ride and the deluxe package which is the bus ride and equipment rental.

First
Since I need to keep track of how many seats are left I figure the best way to do it is to have one item and then put the deluxe package as a modification. I started out with two items but that means i have to manually update the remaining stock every time someone buys a ticket unless there's some way to have two items share the same stock. Is there a way to do that?

Second
I'm a little stuck on how to add the attribute. What i need is to give them the option of the Deluxe upgrade and then if they select Deluxe I will need Height, Weight, Helmet size. I know how to add the Height, Weight, and helmet size but I'm stuck on how to include the upgrade option. Should I do a checkbox or dropdown? Is it possible to link some attributes to another one

Edit: I did figure out how to add the checkbox with cost so I'm good on that half. As an aesthetic choice I'd still be interested in hearing if you think a checkbox or dropdown or whatever would be better for presenting the option.