CartThrob Percentage Option
Note that this was created for a very specific situation, and may not be generally applicable.
If you still want to try: You need to create an item option with a name that ends in "pct". Then make sure each of the possible option names has a number in it, which will be used as the percentage to increase the base price by.
<select name="item_options[gratuity_pct]"> <option value="">Pay gratuity at event</option> <option value="grat_15">Add 15% gratuity</option> <option value="grat_18">Add 18% gratuity</option> <option value="grat_20">Add 20% gratuity</option> </select>