Thanks for your response. Thanks for providing so many pointers.

You mentioned "a loop that finds all flagged products", which requires some query to build a product list first.

Are you aware of some drupal function can do the 'query' (or search) for products (nodes)? node_search() don't look like it.

I hate to use sql query to search the node table in db directly, if I happen to bypass any existing drupal functions, it may make the module less portable. However, if db query is THE way to go, I'll do it.