Can’t use it with other plugins that changes stock management

The plugin does not works when using it along with another plugin that manages the stock (https://github.com/bruno-rodrigues/woocommerce-cart-stock-reducer).

That’s because the way that the stock quantity is retrieved:

$stock_quantity = get_post_meta( $product->id, '_stock', true );

Would be a lot better if the quantity was retrieved via:

$stock_quantity = $product->get_stock_quantity();

That way any other plugin that adds a filter to WC’s get_stock_quantity() will keep working nicely.

Btw, I just submitted a PR with a possible solution. Please take a look at: https://github.com/wp-plugins/woocommerce-product-stock-alert/pull/1

Best Regards,
Bruno Rodrigues