When Your FBA Inventory and Shopify Count Disagree
A customer orders your hero SKU on Shopify. Shopify thought you had twelve. Amazon, where the stock actually lives, had three, and two of those just sold on the Amazon storefront. Now you’re emailing a customer to cancel an order you already charged.
Or the mirror image: Shopify shows zero, so you’ve stopped selling, while a healthy stack of that same SKU sits in an Amazon fulfillment center doing nothing.
Both are the same root problem. When Amazon holds inventory that also feeds your Shopify store, the FBA count and the Shopify count drift apart, and you only find out when one of them is wrong at the worst moment.
Why the two counts drift
If you fulfill Shopify orders out of FBA inventory, you’re running one pool of stock against two storefronts that don’t share a clock.
Amazon is the source of truth for what’s physically in its centers. Shopify shows a synced snapshot of that number, and the sync is periodic, not live. In the gap between syncs, plenty happens that Shopify can’t see:
- Amazon-channel sales decrement the same pool. Ten units sell on Amazon, and your Shopify count is now stale until the next sync catches up.
- FBA adjustments (reservations, removals, damaged or lost units, reconciliation after a count) change Amazon’s number on Amazon’s schedule.
- Multi-channel reservations hold stock for in-flight orders, so “available” on Amazon’s side isn’t the raw on-hand number.
None of that is a bug. It’s the normal behavior of one inventory pool feeding two systems that reconcile on a delay. But the delay is exactly where oversells and phantom stockouts are born.
The two ways it bites
The drift fails in two directions, and both cost you:
- Shopify count too high → oversell. You sell stock that’s already gone. Now you’re cancelling and refunding, and the customer who was excited an hour ago is annoyed. Do it often enough and it shows up in your store’s reputation, not just your support queue.
- Shopify count too low → phantom stockout. You stop selling a product you actually have. This is the same lost-sale problem we covered when stock sits at a 3PL but Shopify shows zero, just with Amazon as the warehouse instead of a 3PL.
It’s worth saying plainly: this isn’t Amazon being slow or your sync tool being broken. Both are doing their jobs. The failure is that nobody is watching the space between the two numbers, which is the same blind spot that shows up whenever a synced count drifts from the source of truth.
What you can actually watch
You don’t need to fix Amazon’s sync cadence. You need to see the disagreement early, from data you already have on both sides:
- The per-SKU delta. Amazon’s available quantity versus Shopify’s available quantity for the same SKU. A persistent or widening gap is the signal, especially on fast movers where it turns into oversells quickest.
- Sync-lag age. How long since Shopify’s count for a SKU last reconciled against Amazon. A number that hasn’t moved through a sales day it should have is suspect.
- Oversell cancellations as a symptom. Track Shopify order cancellations caused by “out of stock” on SKUs FBA fulfills. Each one is a drift event you can trace back.
- Inventory accuracy on FBA-fulfilled SKUs specifically. Your overall number can look fine while the FBA-fed subset quietly drifts.
These are brand-side checks. They don’t require Amazon to hand you anything you can’t already pull from both accounts.
Where this is headed
3PL Pulse already connects to your Amazon account and tracks FBA fulfillment performance: order-to-ship times, MCF delays, whether Amazon is hitting the speed you expect. Watching FBA inventory drift against Shopify is a different question, and it’s one we’re weighing as the next thing to build for multi-3PL brands.
Would you want an alert when your FBA inventory count drifts from Shopify, before it turns into an oversell or a phantom stockout?
We're weighing whether to build this. Your tap helps us decide.
You already chose to sell on both. Seeing the gap should be the easy part
Running Shopify and Amazon off one inventory pool is a smart way to operate. The hard part shouldn’t be guessing whether today’s count is the real one.
Watch the delta between what Amazon holds and what Shopify is selling against, and you catch the drift before it becomes a cancelled order or a sold-out page. Want to talk through how you’d track FBA inventory drift for your store? We’re happy to help.