How to "calculate" one line item total as the whole total?

I have an order form with 18 line items. Each field at the far right is a product of the quantity x the item price. At the bottom right, I have a field that totals all of the line item products, a "grand order total" if you will. The problem: The "grand total" will not calculate unless there are two or more line items. What if a customer only wants to order one item? How can I get that "grand total" to calculate? Thank you so much.