The Storage Math Behind Every FBA vs. 3PL Decision
By
Shannon Curley
Most sellers coming out of this year’s Prime Day events are already deciding how much inventory to push into FBA for Q4. The default answer is often “as much as possible” simply because it’s easier.
In 2026, that default is more expensive, and it compounds faster than it used to.
Amazon tightened the timeline on aged inventory this year, and its capacity system still rations how much room you get based on an IPI (inventory performance index) score most sellers don’t actively manage.
Keeping everything in FBA because it’s the path of least resistance isn’t a neutral choice anymore. Instead, it’s a quickly compounding way to bleed margin … most sellers have never actually run the math on when it stops paying off.
Run that against a typical 3PL’s flat storage rate and a pattern shows up fast: inventory you’re clearing in about 30 days is usually cheapest to send straight into FBA, around 60 days the cost gap closes to near even, and past 90 days a 3PL usually pulls ahead, with the gap widening every month after that.
That break-even shifts with your specific 3PL rate, inbound and outbound fees, and Amazon’s storage utilization surcharges, but the direction holds: fast movers favor FBA, slow movers favor a 3PL.
Most sellers never run this comparison per SKU. They pick a default and apply it to everything, fast movers and slow movers alike, which is exactly how margin quietly disappears on the slow movers.
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Amazon Moved the Deadline on Aged Inventory
The old long-term storage fee used to be assessed twice a year, at 271 days. Amazon’s aged inventory surcharge replaced it with a monthly assessment that starts at 181 days, snapshotting inventory age around the 15th of every month and charging accordingly, on top of your regular monthly storage fee, not instead of it.
TLDR; Inventory that used to have roughly nine months of runway before it triggered a fee now has about six, and the clock runs every month instead of twice a year.
For anything you’re on the fence about, that’s three fewer months to make a call before Amazon makes it for you.
Drop below it and your cubic footage gets capped regardless of how well individual SKUs are selling. Stay above it and you keep more room to work with.
Overstock doesn’t just cost you in storage fees. It costs you in the room you have left to bring in the inventory that actually sells.
What Actually Keeps Inventory From Becoming a Liability
The sellers who avoid these penalties treat stock-out risk and overstock risk as two separate, ongoing checks, not one gut call made a few times a year.
Stock-out risk comes down to lead time against sell-through: how many days of cover are left, and how long it actually takes product to move from a 3PL or supplier into a live FBA listing. Overstock risk is the opposite question, which SKUs are moving too slowly to justify the storage they’re sitting in.
The Tactical Takeaway: Run Your Numbers with Accuracy, Not Guesswork
Q4 volume rewards sellers who already know their break-even point, not the ones figuring it out in the middle of peak. The aged inventory surcharge and IPI-driven capacity limits both got less forgiving this year, which means the cost of guessing went up too.
Run the FBA vs. 3PL break-even on your top SKUs before Q4 volume hits, not after. Check your current IPI score and capacity allocation now, while there’s still time to act on it. And flag anything already past 150 days in FBA before it crosses the 181-day surcharge line. Amazon snapshots inventory age mid-month, so give yourself a buffer rather than cutting it close.
Want a second set of eyes on your FBA vs. 3PL split before peak locks it in? Book a call.
