I am using Zencart and have categories of products. In the category, I have attributes and I have set up an attribute for size. I sell 1 ml, 6ml, 10ml bottles of my product. The 1ml size is a sample size and I would like to limit the number of samples I sell to 30 pieces but I would like to keep the rest of the sizes as an unlimited quantity, so that customers could buy up to 30 samples of the product but after that only full sized versions of the product would be available. How could I do this? Is it possible?