Selling both in a shop and online is where inventory errors multiply. The same unit can be sold at the till and on the website within seconds of each other, and if the two systems are not truly unified, someone gets a cancellation. Here is how POS and ecommerce inventory sync should work on Shopify.
Native unified inventory is the advantage
When you run Shopify POS alongside the online store, both use one catalog and one inventory system. A sale in-store decrements the same stock the website sells from, in real time, with no sync layer to fail. This is the cleanest possible setup and a strong reason to use Shopify POS rather than a disconnected till system.
Multi-location retail
With several shops plus a warehouse, inventory is tracked per location, and the online store shows aggregated availability. This enables omnichannel features like buy-online-pickup-in-store and ship-from-store, where the system needs to know not just how much stock exists but where it is. Getting location logic right is what makes those features reliable.
The third-party POS complication
If you use a non-Shopify POS, you lose native unification and must integrate inventory between the two systems. That means choosing a source of truth, syncing stock and sales reliably, and accepting some latency, exactly the challenges of any inventory integration. It is doable, but it is a build, not a setting.
Watch for the failure modes
The classic problems are sync lag causing oversell on fast movers, and returns or exchanges not writing back correctly. Buffers for popular products, and ensuring in-store returns restock the shared inventory, prevent the most common discrepancies. Reconcile counts periodically so drift is caught early.
Unified data, not just unified stock
The same integration that unifies stock should unify customers, so purchase history and loyalty follow the shopper across store and web. That shared view is the real payoff of omnichannel, covered in retail POS and omnichannel.
Unified inventory turns two channels into one operation. On Shopify, using POS natively is the shortest path to getting it right.