A Shopify stock-sync app keeps the stock figures in your store matched to a source somewhere else: a supplier’s feed, a second Shopify store, a warehouse or a spreadsheet. The Shopify App Store has a whole category of them, and for a store with one supplier and a steady catalogue, one of them is often all you need. They connect by file, spreadsheet or API and update on a schedule, hourly or daily, or in real time between stores.
Where they stop is everything that isn’t that one number. The gap between syncs, stock that should be held back for other channels, bundles made from several products, purchase orders, and silent failures when a feed stops arriving. This piece explains how Shopify itself counts stock, what the apps do with it, where they run out, and when a store needs an order management system or an ERP instead.
What stock-sync apps do
The inventory sync category on the Shopify App Store holds dozens of apps. They fall into a few types.
| Supplier feed sync | Stock, and sometimes prices, from a supplier’s file or API into Shopify on a schedule | syncX: Stock Sync, EZ Inventory, Stockistly |
| Multi-store sync | Stock kept the same across several Shopify stores as orders come in | Syncio, Multi-Store Sync Power |
| Fulfilment apps | Stock held at a third-party warehouse or print-on-demand service | The fulfilment provider’s own app |
Supplier feed sync
These apps read a supplier’s stock file and write the numbers into Shopify. syncX connects to Google Sheets, FTP or SFTP, Google Drive, Dropbox, APIs or custom URLs. EZ Inventory pulls from FTP, SFTP or a web address hourly or daily, and lets you choose whether it updates the Available or the On hand figure. They suit dropshipping and stores that sell a supplier’s stock alongside their own.
Multi-store sync
Apps such as Syncio keep stock the same across two or more Shopify stores, updating when an order, refund, cancellation or restock happens in either. They suit brands running separate stores for different countries or wholesale.
Fulfilment apps
A third-party warehouse or print-on-demand service usually manages its own stock through its app. Shopify’s guide to multi-location inventory says fulfilment apps can manage stock for the same product alongside your own locations: 50 T-shirts in your warehouse, say, and the rest managed by a print-on-demand app.
How Shopify counts stock
Most overselling problems come from not knowing which number an app is writing. Shopify tracks several.
| Available | Stock you can sell: not committed, not set aside, not incoming |
| Committed | Set aside for unfulfilled orders, draft orders, or transfers ready to ship |
| Unavailable | At the location but not for sale: damaged, quality control, safety stock, or held by apps |
| On hand | Everything physically at the location: committed, available and unavailable together |
| Incoming | On its way to the location in a transfer |
Which number the app writes matters
According to Shopify’s guide to inventory states, On hand includes stock already committed to unfulfilled orders. Say the supplier’s feed shows 10 units, and that figure already counts the three units in your open orders. Written as Available, Shopify will sell 10 more: 13 in total against 10 real units. Written as On hand, it leaves 7 available, which is right. If the supplier’s figure doesn’t count your open orders, the opposite is true, so check which it is before switching the app on.
Every location counts separately
Shopify’s multi-location guide says each location keeps its own count and orders are assigned to locations by your order routing settings. A sync app writing to the wrong location, or to one location when stock is spread across three, gives a store-wide number that’s wrong even though each figure looks right.
Safety stock has a place
Shopify’s Unavailable state includes safety stock: units at the location that aren’t for sale. It’s the right place to hold back a buffer for fast sellers, rather than lowering the number the app writes.
Where the apps stop
Sync apps do what they say. These are the things they don’t do, and they’re where the costly problems start.
The gap between syncs
An app that syncs hourly leaves up to an hour where Shopify’s figure and the supplier’s disagree. For a product selling a couple a week that doesn’t matter. For a product selling ten an hour through several channels, the last units can be sold twice before the next sync arrives. Real-time sync narrows the window, but only for the sources that can send changes as they happen.
| 2 a week | 24 hours | Under 1: rarely a problem |
| 2 an hour | 1 hour | About 2: a risk on the last units |
| 10 an hour, across channels | 1 hour | About 10: overselling likely at low stock |
One number, no rules
A sync app copies a quantity. It doesn’t decide that 20 units should be held for Amazon, that the last five go to wholesale, or that a product should stop selling online when the shop floor is down to two. That’s allocation, and it’s the job of an order management system.
Bundles and kits
A gift set made of three products sold separately needs its stock worked out from its parts: the set is available only while all three are. Most sync apps treat each SKU on its own, so bundles either oversell or need a separate app, and sometimes the two apps fight.
Money, purchase orders and the books
Stock figures are only half of inventory. Purchase orders, costs, supplier lead times and the stock value in the accounts live in an ERP or accounting system, and a sync app doesn’t touch them. When those are needed, see the ERP integration page and which ERP connector to choose.
Silent failures
If a supplier’s feed stops updating, or changes a column name, some setups carry on with the last numbers they had. Nothing looks broken until customers order stock that isn’t there. Check that the app alerts you when a feed is late or malformed, and that someone receives the alert.
When a store needs more than an app
An app is the right answer until the rules around the stock number matter more than the number itself.
◆ App or system
A sync app is enough when
- Stock comes from one or two sources
- Products sell at a steady, modest rate
- Shopify is the only or main sales channel
- No bundles, or very few
- Purchase orders and stock value live elsewhere and that’s fine
You need an OMS or ERP when
- Several channels share the same stock
- Stock must be allocated or held back per channel
- Bundles and kits are a real part of the range
- Purchase orders, costs and stock value must match the store
- Overselling has already cost refunds and reviews
Marketplaces
Selling the same stock on Amazon, eBay or TikTok Shop as well as Shopify is where single-number sync breaks down fastest, because every channel sells from the same shelf at once. The marketplaces page covers connecting them properly.
Warehouses
A third-party warehouse or 3PL usually holds the truth about stock. Syncing from its system, rather than from a spreadsheet updated by hand, removes a whole class of errors. The warehouse and fulfilment page covers the options.
Forecasting
Once stock figures are reliable, the next question is how much to reorder and when. That’s a forecasting job, covered in AI demand forecasting and stockouts.
Choosing and setting one up
If an app is the right answer, a little care at the start avoids most of the problems above.
- Check the sync frequency against how fast your quickest products sell.
- Decide which figure it writes, Available or On hand, and test it with open orders in place.
- Map every location the app writes to, and check order routing sends orders where the stock is.
- Set safety stock on fast sellers using Shopify’s Unavailable state.
- Turn on alerts for late or failed feeds, and send them to someone who reads them.
- Test with one supplier and a handful of products before connecting the whole catalogue.
◆ Glossary
- Stock sync
- Copying stock figures from one system into another on a schedule or in real time.
- Committed stock
- Units reserved for orders that haven’t been fulfilled yet.
- Safety stock
- A buffer held back from sale so fast sellers don’t sell out between deliveries or syncs.
- Allocation
- Deciding how much of the same stock each sales channel is allowed to sell.
- OMS
- An order management system: it routes orders and allocates stock across channels and locations.
- 3PL
- A third-party logistics provider that stores and ships stock for you.
◆ Sources
