People ignoring minimum order

Hi,
I sell products priced to include postage so do not have shipping costs set up. Although it clearly states on my home page that I have a £12 minimum order requirement, people are just ingoring it and I am getting lots of costly small orders. I don't mind shipping small orders if people add £1 per item to cover the extra costs, but emailing them and getting it out of them is timeconsuming and annoying. Can I set up the billing page either so orders under £12 can't go through, or with an option to add the extra postage or preferably both?
Margaret

Install this module:

http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_60&products_id=599

I have it working at www . edutrade . co . uk where the minimum order is £8.

Shopping carts with under £8 cannot proceed to checkout.

Or you could use the low order fee module under includes->modules->order total and tack on an extra bit if the order is below your threshold.