Item with attributes that don't affect stock

[Astro1176]

I am an author selling books that I have written. I am really impressed with Prestashop so far. I hold stock from the publisher, and can offer them signed or unsigned (in this case I offer this at no cost).

 

I have set this up as an attribute of each book, but it seems I can't set up a stock level for the book - I have to set a stock for signed books and a stock level for unsigned books. Is there a way to reflect the reality better - and just have one stock level which is decremented, whatever sort of book is purchased?

 

 

 

As another point related to the attribute, I would prefer the default to be undefined and for the buyer to actively have to pick either 'signed' or 'unsigned'. Is there a way to do that too?

 

 

Thanks for any help on this.