Add custom rule for minimum Order Total

Is there a way to have a custom rule for setting a minimum order total. i.e. I want to set a minimum order total of 15$ per order HOWEVER I want to be able to accept orders containing a sample product (priced at 0$).

So the rule would be:

If the order total = 0$ AND productSample is not in order THEN display minimum order total warning
ELSE
Display the CONFIRM ORDER button

Help please!

Thanks