Don't sell if shipping cost is 0€

[osonabit]

Hi,

I have a problem, and i need some help, please.

I have a WebService with the shipping company that returns to us the cost of the shipping of each order, but sometimes if the customer doesn't introduce the Postcode correctly, the shipping cost is 0€. I can't check each postal code, because we work with the majority countries of europe and it's unworkable, and I'd like know if there is some plugin that doesn't allow check out the order if the shipping cost is 0€.

Another possibility is make some hack to the code to check is the shipping cost is 0€ before the checkout o just when the costumer click on Checkout, but I don't know which files I have to modify.

Can somebody help me, please?

Thanks.