Hi,
The display limited to option is indeed the solution for that. You need to check the checkbox in that option of the field so that it appears only with the checkbox is checked.
For your second question, there is no option to add a cost to the product basedon the value of a field.
That would require the creation of a small hikashop plugin to override the quantity price calculation. You can see an example of that with our donation plugin:
demo.hikashop.com/index.php?option=com_h...ct&task=show&cid=214