Does Stocky do what you need? https://apps.shopify.com/stocky
It's Shopify's own inventory management app, handles multiple locations and works with Shopify POS.
Would also recommend checking out TrueStock (https://apps.shopify.com/truestock). It auto-syncs every day and predicts your sales demand using machine learning then notifies you automatically when you need to replenish or have overstocked. You can also provide it with your stock lead times and it will alert you giving you enough time to create a purchase order and never run out of stock. It integrates with slack too! Hope this helps.