Coupon should not be purchased as $0.00 dollars.

In coupon module, if a coupon is set as the type where customers enter/nominate the coupon amount, then it is possible that a coupon be bought for $0 dollars.

This is undesirable as it just clogs up the transactions list with no income at all. Plus, the potential for confusion.

*It seems that validation does not work on coupons.