Business rule to prevent orders with zero price

I'm sure this must be simple but I'm just missing it.

We have an issue here where people keep raising orders for £0.00. I thought that I could create a simple business rule to prevent this.

I created a rule that said the field "total amount" must not be zero and to display an error message. This works perfectly. However, to add product lines to the order you must save the order which we now cannot do.

I'm sure this should not be too complicated. Just something simple to remind people to put an amount in.