I have a new site with a custom node type--each node is also a product for the ecommerce. Each node is assigned a taxonomy term. We want to monitor the number of nodes of each taxonomy term which are not sold. The site owner wants to receive an email if the number of available 'products' falls below 25 so that he can make more.

This is not an inventory issue--each product only has a quantity of one--this is a 'catalog' issue.

I figured the only solution is a little custom module that checks that number of products available in each category and if it's < 25, send an email.

Is that the right way? If so, are there any nice API calls I can use to make this check? I am not a real Drupal expert so my first thought of course it just to write the raw SQL, but I imagine there is a far more elegant approach.

Thanks, Fred