Sync Stock: Shopify, Amazon and Physical Shop

The real problem: out-of-sync stock during peak season

A clothing shop in Fuengirola or a gourmet food distributor in Málaga faces exactly the same problem as a large chain — just with less margin to absorb the errors. During tourist season, online orders and in-store sales spike at the same time. If the system isn't prepared, mistakes happen at the worst possible moment.

The most common symptoms of out-of-sync stock are:

Overselling: the same item sells on two channels when only one was left. One order has to be cancelled, with all the time, logistics and reputational cost that brings.
Ghost stock: the system shows a product as available when it actually sold out days ago because someone sold it in-store without updating the ERP or the ecommerce.
Unnotified pending orders: a customer buys online, the product is out of stock, and nobody tells them until they chase the order status.
Badly managed in-store reservations: a customer reserves a product in the shop, but it keeps showing as available online.

On the Costa del Sol, where tourism creates sharp demand spikes concentrated in short windows of time, these failures aren't edge cases — they can happen multiple times in a single August afternoon.

Before vs after: isolated channels vs single source of truth

Without sync

✕Shopify: 3 units (own counter)
✕Physical shop (POS): 3 units (own counter)
✕WooCommerce: 3 units (own counter)
✕Sale on one channel → others don't know
✕OVERSELLING inevitable during peak periods

With sync

✓Central inventory: 3 units (single source)
✓Sale on any channel → all read 2 units instantly
✓Stock hits 0 → all channels show "out of stock"
✓No manual intervention needed
✓Zero overselling during peak season

Isolated channel counters cause overselling; a single source of truth eliminates it

What a "single source of truth" is and why it changes everything

The single source of truth is a simple principle: one system decides how much real stock exists for each product, and all sales channels consult that system before confirming any transaction.

Without this principle, each channel has its own stock counter. Shopify has one, Amazon has one, the in-store POS has one. When a sale happens on one, the others don't know about it until someone updates them manually — or until it's already too late.

With a single source of truth:

There is a central inventory (your ERP, Shopify as master system, Odoo, or even a well-connected spreadsheet).
Every sale on any channel immediately deducts from that central inventory.
All channels receive the update automatically, with no manual intervention.
When stock hits zero, every channel marks that product as out of stock simultaneously.

Practical key: The single source of truth doesn't need to be the most expensive system. What matters is that it's the central node every channel reports to and reads from. Shopify can serve this role if it's properly integrated with your physical shop and with Amazon.

How real-time sync works: webhooks and APIs

Manual sync — exporting a stock spreadsheet every morning and uploading it to each channel — works at very low volumes. For any business with more than a handful of SKUs and daily transactions, it's a continuous risk.

The alternative is automatic sync through webhooks and APIs:

Webhooks

A webhook is an automatic notification that one system sends to another when something happens. When Shopify records a sale, it can automatically send a notification to your central system saying "2 units of SKU 4821 just sold". The central system updates its inventory and in turn notifies Amazon, the in-store POS and any other connected channel.

APIs

APIs let different systems talk directly to each other. Shopify has a well-documented public API. Amazon Seller Central also has one, though with more restrictions. Odoo has its own API. A middleware layer — which can be n8n, a custom script, or an integration platform — acts as translator between all of them.

The end result: when a customer buys something at your Marbella shop and the POS registers the sale, that transaction automatically flows to the central inventory, which deducts the units and updates the stock on Shopify and Amazon within seconds.

Real tools for building this system

There's no single right answer. It depends on volume, number of channels and budget. These are the most common options:

Shopify as master system

If Shopify is already your main ecommerce platform and you also sell on Amazon, you can use Shopify as the single source of truth. Shopify natively supports multiple inventory locations (physical shop + warehouse + additional point of sale). Integration with Amazon is done via the official Amazon app on Shopify or through third-party connectors. The weak point is the physical shop: you need a Shopify-compatible POS (Shopify POS) to close the loop.

Odoo

Odoo is an open-source ERP with inventory, ecommerce, point of sale and Amazon connector modules. For businesses with greater logistical complexity — multiple warehouses, batch management, product traceability — it's the most robust option without needing to pay for a proprietary ERP licence. The initial setup takes more work, but the result is a centralised system that controls the entire flow.

WooCommerce + sync plugins

If your ecommerce runs on WooCommerce, there are specific plugins for syncing stock with Amazon (such as WP-Lister) and with physical management systems. Quality varies significantly between plugins, so testing before relying on them in production is essential.

n8n as integration middleware

n8n is a workflow automation tool that connects systems that don't have native integration with each other. Instead of paying a SaaS suite for multichannel inventory management — which can cost several hundred euros a month — you can build custom sync logic with n8n: when a sale is registered on the POS, n8n receives the event, deducts from the central inventory and updates Shopify and Amazon via API. n8n can also handle the low-stock alerts covered below.

Low-stock alerts and automated replenishment

Stock sync is half the job. The other half is not running out of product at the wrong moment. That's where low-stock alerts and replenishment flows come in.

A well-configured system does this:

Defines a minimum threshold per SKU: for example, when fewer than 5 units of a product remain, an alert fires.
Sends the alert to the right place: this could be an email to the purchasing manager, a Slack or WhatsApp message, or a task in the management system.
Pauses online sales when stock hits zero: instead of continuing to accept orders for an out-of-stock product, the system automatically marks it as unavailable across all channels.
Generates a replenishment order: if you have integration with your supplier or distributor (via Sendcloud or another logistics system), the flow can generate a replenishment request automatically when stock drops below the threshold.

This kind of automation saves management time and removes the human error of forgetting to check the stock of a specific product during the chaos of peak season.

Common mistakes when syncing multichannel stock

After seeing this implemented across various retail businesses, these are the problems that come up most often:

One-way sync: Shopify is connected to Amazon so Shopify sales update Amazon, but not the other way round. Amazon sales don't update Shopify. Result: overselling on Shopify with stock that already sold on Amazon.
Leaving the physical shop out of the system: ecommerce syncs perfectly, but in-store sales aren't recorded in the central inventory. The physical shop remains the black hole of your stock.
Inconsistent SKUs: the same product has a different code in Shopify, Amazon and the management system. The sync fails silently because the systems don't recognise each other. Standardising SKUs before integrating is a step that cannot be skipped.
Not accounting for returns: when a customer returns a product, does it automatically go back into available stock? Or is there a review process first? If this isn't defined, returns create inventory discrepancies that accumulate over time.
Assuming native integration is enough: official connectors between platforms (Amazon to Shopify, for example) often have limitations: slow sync frequency, limited support for product variants, no support for multiple warehouses. Always review what the connector actually does before trusting it.

One-way vs two-way: the most common mistake

One-way (broken)

✕Shopify → updates DHL/ERP but Amazon doesn't update Shopify
✕Physical shop sales never reach the ecommerce
✕WooCommerce and Shopify have different counters
✕Silent overselling discovered too late

Two-way (correct)

✓Every sale on any channel flows to central inventory
✓Central inventory propagates to all other channels
✓Physical shop connected just like online channels
✓One single counter of truth, no discrepancies

One-way sync without return is the most common hole in multichannel retail

Checklist: multichannel stock sync

Before signing off on the system, verify these points one by one:

A clearly defined single source of truth exists (one master inventory system).
All sales channels — including the physical shop — deduct from the same central inventory.
SKUs are identical across all systems (or a mapping table exists).
Sync works in both directions: online sales update the physical shop and vice versa.
Low-stock alerts are configured with thresholds per SKU.
Products automatically go out of stock on all channels when inventory hits zero.
Returns have a defined process: what happens to returned stock before it goes back on sale.
The system has been tested with real sales before going live in production.
There is a stock change log to audit discrepancies.

Checklist: before going live in production

✓A clearly defined single source of truth (one master inventory system)
✓All channels — including the physical shop — deduct from the same central inventory
✓SKUs are identical across all systems (or a mapping table exists)
✓Two-way sync verified in both directions
✓Low-stock alerts configured with per-SKU thresholds
✓Products automatically go out of stock on all channels when inventory hits zero
✓Returns process defined before stock goes back on sale
✓System tested with real sales before going live
✓Stock change log available to audit discrepancies

Key points to verify before activating sync in production

Do you have a physical shop and sell online too?

At Zerolagia we build multichannel inventory integrations for retail on the Costa del Sol: Shopify, Amazon, WooCommerce and your in-store POS all in sync, with alerts and zero overselling. No expensive SaaS suites required.

See how we do it

Frequently asked questions

What happens to Amazon FBA stock when I sell on other channels?

Amazon FBA manages its own physical inventory in Amazon's warehouses, separate from the stock you hold in your own shop or warehouse. Multichannel sync must treat FBA as an independent warehouse: when someone buys on your Shopify, the system should only deduct from the stock you control, not from FBA, and vice versa. If you sell on Amazon Seller Central without FBA, real-time sync against a centralised inventory is straightforward.

Is real-time sync better than batch sync?

It depends on your sales volume. For shops with heavy traffic during peak season, real-time sync via webhooks is the only safe option — any delay can cause overselling. For shops with only a handful of daily transactions, batch sync every 15 or 30 minutes may be enough and is simpler to maintain. A practical rule of thumb: if you sell more than 20 units a day on a channel, use real-time.

How are returns handled in a centralised stock system?

Every return should trigger a stock update in the single source of truth, just like a sale but in reverse. The complication is that a returned product isn't always immediately sellable again — it may need inspection, repair or liquidation. A well-designed system distinguishes between available stock, stock under review and damaged stock, and only pushes genuinely available stock back to the sales channels.

Can you manage stock across multiple warehouses or locations?

Yes, though it adds complexity. Shopify natively supports multiple inventory locations. For a true multi-warehouse setup you need to define priority rules: which warehouse fulfils which order based on customer location or availability. Tools like Odoo have built-in multi-warehouse management. With n8n you can orchestrate the order routing logic between locations without paying for a full ERP licence.

Can n8n replace a paid inventory management suite?

For many mid-sized shops, yes. n8n acts as the glue between Shopify, Amazon, WooCommerce and your management system, automating stock updates, alerts and replenishment without paying monthly SaaS licences that can cost hundreds of euros. For businesses with high logistical complexity — dozens of warehouses, thousands of SKUs, complex reverse logistics — an ERP like Odoo is worth evaluating.

How to Sync Stock Between Shopify, Amazon and Your Physical Shop Without Overselling