It looks like you have the calculation all setup for the gift wrap column. It looks like you just need to get the checkbox to add the price of the gift wrap to the cart with the item.
To do this you should add a check box to the add to cart form. For the value you can set it to the price of the gift wrap. Then in the add to cart server behavior you would bind this checkbox to the giftwrap column.
The way that the checkboxes work they will only post what is in the value attribute if the box is checked. If not it will not post a value. So binding the cart column to the form element directly will ensure that a value is bound only if the box is checked.
If you make this an updatable checkbox in the cart it should allow you to toggle the gift wrap option.
In the checkbox property inspector the 'check if' is for dynamic checkbox's that you would like to be checked based on a value. The equal to box is part of this, so if you had a recordset with a column value of 1 and you wanted the checkbox to default to be checked based on this column you would enter the details into here.