A customer can modify the quantity of an item on a quote if you specify the item on the quote, and also add the item to "Optional Products" on the quote. Note that only the items which are listed on both the optional products and order lines can have their quantity adjusted. Products on Order Lines, but not optional products, are not adjustable; products on Optional Products, but not Order Lines, are not added to the quote but are available optionally to be added by the customer.