Good news, we’ve just launched basic inventory management in the last update. Head on over to Settings > Products and give it a whirl.
Once a product appears in an Invoice it will be removed from the available quantity. (if that Invoice is deleted the available stock Quantity will come back)
This has been our most requested feature for a long time. We’ve designed it in a way to facilitate future product related features ( such as: reporting profit from product sales, re-ordering out of stock products, front end e-commerce for purchasing certain products directly from UCM, etc.. )