Except minimum order amount from free products

Hi

I did a shop with a minimum order amount of 20.00.

For that I used this code:

http://docs.woothemes.com/document/minimum-order-amount/

It works well but now the problem is that the owner of the shop has set some products for free (as product sample). Now the problem is that when someone just order a freebie that the system says that the minimum amount is not reached. But the owner of the shop wishes that customers just can order a freebie without other products if the want to.

How can I do that?

https://wordpress.org/plugins/woocommerce/