The short answer
Updating stock by hand on the till and the website separately works fine for about a week, then someone forgets, and a customer orders something that sold on the shop floor an hour earlier. The fix is one shared stock list that both the till and the website read from, which happens in one of three ways: the till and website run on the same platform, a sync app bridges two different systems, or a custom link is built between them. Which one suits you depends mostly on whether your till and your online shop are already from the same vendor.
Why updating both by hand stops working
It starts well. You count stock once, enter it on the till and on the website, and for a week or two the numbers roughly hold. Then a Saturday gets busy, six things sell in person that nobody logs online that evening, and by Sunday the website is quietly offering four items that don't exist. The customer who orders one of them isn't being unreasonable. They checked the page, it said in stock, and now you're writing an apologetic email and either refunding them or chasing a replacement you may not have.
This isn't really about discipline. It's a structural problem: two separate numbers, updated by a human, in two separate places, will drift apart the moment either place gets busy. The fix isn't "try harder to remember." It's removing the second number entirely.
One stock list, two places to sell
The aim is a single source of truth for how many of something you actually have. The till reads from it when something sells in person. The website reads from it when something sells online. Neither place keeps its own private count. When that's in place, a sale anywhere updates availability everywhere, and the oversold-item email stops happening, not because anyone's more careful, but because there's only one number left to get wrong.
Same system, a sync app, or a custom link
Same platform for both. If your till and your online shop are the same vendor (Square till with Square Online, or Shopify POS with the Shopify website), this is usually already built in, and the work is mostly making sure it's actually switched on and configured correctly rather than left on default settings. This is the simplest route when it's available, covered in more detail in our comparison of Shopify, Square and WooCommerce for a shop with a till.
A sync app or plugin. If your till and website are from different vendors, a dedicated sync app can connect the two, polling stock levels on a schedule (sometimes near-instant, sometimes every few minutes) and pushing changes both ways. This works well for straightforward product catalogues and is usually the quickest route when "same platform" isn't realistic without a bigger change.
A custom link. When the off-the-shelf sync apps don't cover your specific setup, often because of how your products are structured or because one of your systems doesn't offer a ready-made connection, a direct integration built between the two systems is the remaining option. This costs more and takes longer than an app, so it's worth ruling out the simpler routes first.
What usually breaks sync
A few things catch shops out even once sync is set up. Variants (the same jumper in three sizes and four colours) need every variant tracked separately, not just the top-level product, or you'll oversell one size while another sits in stock. Bundles (a gift set made from three items you also sell individually) confuse a simple sync unless it's told how bundling affects each component's count. Items sold at a market, pop-up or second location that isn't connected to the main system at all are the classic gap: nothing's wrong with the sync, there's just a sale happening somewhere the sync can't see. And a sync that only runs every few minutes, rather than near-instantly, can still leave a short window where overselling is possible during a genuinely busy rush.
What an oversell really costs you
Beyond the refund or the replacement item, an oversold order costs you an apologetic email, a delay the customer didn't sign up for, and a bit of trust that's harder to rebuild than the stock count itself. It also costs staff time: someone has to notice, explain, and sort it out, usually at the worst moment to be distracted. None of that shows up as a line on a spreadsheet, which is partly why it's easy to underestimate how much a flaky sync is actually costing a small shop. It's also exactly the kind of quiet, upstream problem our general guide to a shop's website not converting visitors into orders points to: the friction often starts before checkout, not on the checkout page itself.
When to get help, and what we'd do
If you're setting up an online shop for the first time, getting stock sync right from day one is part of our web-shop component: your till connected properly where that's realistic, variants and bundles accounted for, rather than bolted on as an afterthought once problems start. If you've already got a shop and a till from two different vendors that genuinely need to talk to each other reliably, that's usually our auto-integration work, scoped after a short call once we understand both systems and how your products are actually structured. See our automation service for how that kind of project typically runs.
Frequently asked questions
How quickly does stock sync usually update?
It depends on the route. Same-platform syncing (Square till with Square Online, for example) tends to be close to instant. A sync app usually checks on a short schedule, often every few minutes, which is normally fine unless you're genuinely busy on both channels at the same moment.
Can I sync stock between Etsy, my till and my own website all at once?
Sometimes, but it gets more fragile the more systems are involved, because each connection is another place sync can silently break. We'd usually recommend treating your own website and till as the primary pair to get right first, then looking at whether Etsy needs to be part of the same system or kept separately with manual checks.
What happens if the sync breaks without anyone noticing?
This is the main risk with any automated sync: it works quietly until it doesn't. A good setup includes some way of noticing when sync has stopped, rather than finding out from an angry customer, which is something we build in rather than leave to chance.
Do I need a custom integration, or will an app do the job?
Most straightforward product catalogues are fine with an existing sync app. A custom link is usually only worth the extra cost when your products have unusual structure (heavy use of bundles, made-to-order items, multiple sales channels) that off-the-shelf apps weren't built to handle well.
Is this worth doing if I only sell a handful of products online?
Even a small catalogue can oversell if a popular item sells out in person without the website knowing. The smaller the catalogue, the simpler the fix usually is, so it's often one of the quicker, cheaper problems to solve properly.