Keeping Stock in Sync Across Channels
By CodexierPublished 5 min read
Selling in your own webshop, on a marketplace and in a physical store multiplies your reach, and your ways to sell the same item twice. Overselling means a cancelled order, an apology and, on marketplaces, a penalty to your seller rating. This guide explains the master-inventory principle, how fast sync needs to be, and when a ready-made tool is enough versus when a custom integration pays off.
One master inventory
The principle is simple: exactly one system knows the true stock level, and every other channel is a window onto it. That master can be your e-commerce platform, such as Shopify with its point-of-sale, your ERP or accounting system if it manages stock, or a dedicated inventory tool. What matters is that stock is never adjusted by hand in two places. The moment someone updates a quantity directly on a marketplace, the master is no longer the master.
Choose the master based on where stock is received and counted. If goods arrive in a warehouse managed in your ERP, that is the natural master. If you are a small shop running everything in Shopify, Shopify is the natural master and the other channels connect to it.
Sync timing and overselling
Overselling risk depends on two things: how many units you hold per item, and how often that item sells. An item with one unit that sells in two channels can be oversold in the minutes between syncs. An item with fifty units that sells a few times a week will never be a problem with an hourly sync.
| Situation | Sync method | Buffer |
|---|---|---|
| Unique items or one-offs | Real-time on every sale (webhooks) | None needed if real-time works; otherwise list on one channel only |
| Fast sellers, low stock | Real-time | One or two units on slower channels |
| Steady sellers, good stock | Every 5 to 15 minutes | Small buffer on marketplaces |
| Slow sellers, deep stock | Hourly or nightly | Usually none |
The technical difference between real-time and scheduled sync is explained in webhooks or scheduled sync. Real-time is better but depends on every channel sending events reliably, so even a real-time setup should run a periodic full reconciliation to catch missed events.
Marketplaces and physical stores
Marketplaces such as Amazon, CDON and Fyndiq each have their own APIs, update speeds and rules for cancellations, and a cancelled order hurts your standing there more than in your own shop. That is why a buffer on marketplaces is common: show one or two fewer units than you have. The trade-off between marketplace reach and your own shop is covered in marketplace or own webshop.
- A physical store must register every sale in the till system that talks to the master; manual sales on paper break the chain.
- Items on display or reserved for a customer in the store need a status, or they will be sold online.
- Stock counts should be done in the master system, not in a separate spreadsheet.
Tools vs custom integration
For many stores a ready-made multichannel tool or marketplace connector is the right answer. It is quick to set up and maintained by someone else. Custom integration becomes worth it when the ready-made tools do not support your combination of systems, when you have product variants or bundles that tools handle badly, or when the monthly fees scale with order volume to the point where owning the integration costs less.
| Factor | Ready-made tool | Custom integration |
|---|---|---|
| Time to start | Days | Weeks |
| Cost profile | Monthly fee, often scaled by volume | One-off build plus maintenance |
| Bundles and variants | Often limited | Built for your rules |
| Your systems | Only supported ones | Any system with an API |
Handling reservations and returns
Two rules cause most of the remaining confusion. When is stock reserved: at order, at payment, or at shipping? For card and Swish payments, reserve at order; for invoice payments that can fail, reserve at order and release if the payment is rejected. And when does a returned item come back into stock: only after it has been inspected and approved for resale, never automatically when the return is registered.
When not to build anything: if you sell in one channel, or your second channel has deep stock and slow sales, a nightly sync or even manual updates will do. When you oversell regularly or your systems do not talk to each other, our custom e-commerce features cover exactly this kind of integration. Book a short call and bring a list of your channels and systems.
Frequently asked questions
Which system should be the master inventory?
The one where stock is received and counted. For a small shop that is often the e-commerce platform; for a business with a warehouse it is usually the ERP or inventory system.
Is real-time sync always necessary?
No. It matters for unique items and fast sellers with low stock. For slow-moving goods with deep stock, a scheduled sync is simpler and just as safe.
Why do I still oversell with a sync tool?
Usually because stock is adjusted manually somewhere other than the master, a channel fails to send events, or bundles and variants are mapped wrongly. Start by checking where manual changes happen.
Can Fortnox be the master for stock?
Fortnox has stock management that can act as master for simpler setups. For many variants or channels, a dedicated inventory system or your e-commerce platform may work better.
Selling the same item twice?
Bring a list of your channels and the systems behind them. On a short call we will identify the right master, the sync timing you need, and whether a tool or a custom integration fits.
Book a free 15-minute call