It would be nice to be able to say a base cost for an item shipping (say $5) and then a cost for each additional ($.25) so when selling multiple small items the shipping doesn't get out of hand. Another option for this would be a weight/size based calculation using Shippo now, which is more advanced for sure but would give a much more accurate rate.

But for the former, a new text box (or two) for the "additional" cost could just override the default calculation. So if the box was empty it would be the base price * item quantity. If the box was 0 then it would be just the base price no matter quantity of items purchased. If the box was != 0 then it would be base price + (quantity * additional cost). One for domestic and one for international would be the most obvious, I would think.