How to implement *alternative* attributes / variants / combinations for the very same product?

[Pippo3000]

folks, what is the best way if any to offer alternative 'attributes' (if they are still called this)? I want to sell say 10 bean bags and offer them with filling (higher price) and without (lower price). now I was just about to add a combination when I realized that I have to define amounts per combination, i.e. I could add 10 without and 10 with filling. But then PS adds them to 20 in total which is wrong. of course I can just add 5 each which would total to 10. but is there a more elegant option to tell PS that I have 10 items and that a customer can still make a choice and PS substracts from the overall amount of 10?

Workaround would be to sell the filling as separate item. but do I really have to go that path?

phil