NO SUPPORT FROM DEVELOPER
Necessity for C5 eCommerce!
Concrete5 core commerce does NOT support quantity updates, the field exists but as noted in the core commerce video it does NOT update on when a purchase is made! I would not have been able to use c5 or had to write something on my own if not for Product Multi Inventory Addon. And I haven't even gotten into the multi option part of it!
If you need to make sure that you don't oversell a product you need this addon.
Now if we could only have a different price for each option... ;)