This aims to solve the problem of dealing with WooCommerce variable products, where only one is subtracted from the total product inventory. The use case I had for it was as follows: Customer wants to sell packs of cigars 3/5/10 and wants each variation to be subtracted from the main product total. So I built this simple plugin to manage variable product inventory and dynamically update product quantities, using woocommerce. Basically it doesn't solve this question mine, but it illustrates the point: http://wordpress.stackexchange.com/questions/72662/woocommerce-fixed-quantity-of-a-product.