I assume that you're setting the "productListItems" field in the Web SDK hit's XDM object.
For each item object in the "productListItems" array, there is a "priceTotal" field. As the field name implies, you should be using the total value of the items for the tracked quantity. For example, if your item's unit price is $1.99 and the user buys 2 units, then you should track 3.98 in this "priceTotal" field.
I'm also not sure what you mean by "value+value". Can you provide an example of what you mean?