Can a coupon reduce the price below zero for a credit?

A coupon code can reduce the price to zero, but not below. The codes do not work like gift cards; once used, any remaining "balance" is simply lost.

Example: 

A single use Personal Coupon code for "$50 Off" is used on an order form for a product that costs $10:
- Net cost of the product is $0
- There is no balance left on the coupon and it expires with the first use