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Order Matrix? (multiple items at once before adding to cart)
That`s not possible, out of the box.
You have to write an own extension or just modify the list - template to add the \"size choose\” - options\” in the product list view behind the qty - field and put a global \"add to cart\” - button in this view...so the user can scroll through the products on this site, choosing the size, the qty and so on, then click the new global \"add to cart\” - button and all products with all variants will be added in the cart.
But of course, you have to develop it. Or write me an email if you need help.