This is no limit to the number of cart columns you can add.
You should have a cart column named giftwrap, set the checked value for this column to 1
on the charges tab, create a new charge as:
Trigger:
"Number Of Items With A Specific Column Value"
If the Total Number of items where "giftwrap" has a value of "1" is ">" a value of "0"
calculation:
"Based on items with a specific value"
Total of "Quantity" where "giftwrap" has a value of "1" "times" "3.95"