Quick answer: FBA converts fulfillment into predictable per-unit fees and takes the delivery metrics off your plate, but adds storage and aged-inventory surcharges that punish slow sellers. FBM keeps fees low and control high, at the cost of owning shipping speed, tracking, and the performance metrics Amazon watches.
The FBA-versus-FBM decision usually gets made once, at launch, and then never revisited even as a catalog shifts. That's a mistake, because the math changes item by item and season by season. It's worth running the comparison per SKU, not per account.
What FBA actually costs
FBA's headline number is the fulfillment fee, charged per unit based on size tier and weight. That single fee covers picking, packing, shipping, customer service, and standard return handling — genuinely a lot of labor for the price on small items.
Around it sit the fees that catch people out:
- Monthly storage, charged per cubic foot, higher in Q4.
- Aged-inventory surcharge on units sitting past roughly six months, escalating the longer they stay. Covered in the aged-inventory surcharge guide.
- Low-inventory-level fee on FBA units when your weeks of cover run thin, explained in the low-inventory-level fee post.
- Inbound placement fees for sending stock to fewer receive centers, in the inbound placement fee breakdown.
- Returns processing fees in certain categories, per the returns processing fee post.
The pattern: FBA's base fee is fair, and the surcharges all penalize inventory that doesn't move. A SKU turning every three weeks barely feels them. A SKU turning twice a year gets buried. The full 2026 picture is in the FBA fees guide.
What FBM actually costs
FBM's costs are your real costs: postage, packaging, the labor to fulfill, storage wherever you keep the stock, and the staff time to handle returns and customer messages. There's no Amazon fulfillment fee and no FBA storage or surcharge exposure.
The catch is that these costs are yours to control and yours to get wrong. Negotiated carrier rates make FBM competitive; retail postage rates don't. A tight packing operation keeps it cheap; a chaotic one doesn't. And every hour spent packing boxes is an hour not spent on the parts of the business that grow it.
FBM tends to win on: large and heavy items where FBA's fulfillment fee is punishing, slow sellers where storage and aged-inventory surcharges would accumulate, very low-margin items where any per-unit fee erases the profit, made-to-order or high-variation catalogs, and hazmat or oversize goods FBA handles poorly. AWD, FBA, and 3PL covers the hybrid setups.
The metrics you inherit with FBM
This is the part that doesn't show up in a fee spreadsheet. Under FBA, Amazon owns every delivery metric. Under FBM, these become your responsibility, with thresholds that matter:
| Metric | Rough target | What breaks if you miss it |
|---|---|---|
| Late Shipment Rate | under 4% | Featured Offer eligibility, account health |
| Valid Tracking Rate | at or above 95% | Featured Offer eligibility |
| On-Time Delivery Rate | high 90s | Featured Offer eligibility |
| Cancellation Rate (seller) | under 2.5% | Account health |
The seller-fulfilled metrics guide goes deeper on each. Miss them consistently and you don't just lose the box — you can lose the ability to sell.
The Buy Box tax on FBM
The Featured Offer calculation weighs delivery speed and reliability heavily. An FBA offer, or a Seller Fulfilled Prime offer, arrives with a fast-shipping badge and a strong track record baked in. A standard FBM offer at the same price competes from behind. On a listing you share with other sellers, that gap can mean winning the box 40% of the time instead of 90% — and a session-weighted view will show you exactly how much that costs, since the blended percentage hides it.
If FBM is right for the economics but the Buy Box loss is too expensive, Seller Fulfilled Prime is the bridge: FBM control with a Prime badge, in exchange for meeting strict, sustained performance bars.
How one seller split the catalog
A homewares seller with about 200 SKUs ran everything FBA at first. Reviewing storage and surcharge reports at year-end, they found a cluster of large, slow ceramic pieces — maybe 15 SKUs — quietly eating the account's aged-inventory surcharge. Those moved to FBM with a regional 3PL and negotiated freight. The fast-moving 80% of the catalog stayed FBA. Total fulfillment cost dropped, the Buy Box loss on the FBM items was minor because they were the sole seller, and the surcharge line nearly disappeared.
That's the real answer: it's rarely all-or-nothing. Sort SKUs by size, velocity, and margin, and let each one land where its own math points.
EcomSanity's inventory and aged-inventory views show which SKUs are turning fast enough to justify FBA and which are accumulating storage risk — the sort you need before moving anything to FBM. See the days-of-stock alert that actually matters.
Frequently asked questions
Is FBA or FBM cheaper?
For small, light, fast-moving items FBA is usually cheaper per unit once you count what it would cost you to pick, pack, ship, and handle returns yourself. For large, heavy, slow-moving, or low-margin items, FBM often wins because FBA's fulfillment fee, storage, and aged-inventory surcharges scale against you.
Does FBM hurt my Buy Box chances?
It can. The Featured Offer calculation rewards fast, reliable delivery, so an FBM offer with standard shipping competes at a disadvantage against FBA and Seller Fulfilled Prime offers on the same listing, all else equal.
What metrics do I take on with FBM?
Late Shipment Rate, Valid Tracking Rate, Cancellation Rate, and On-Time Delivery Rate become yours to manage. FBA absorbs all of those. Missing the thresholds risks the Featured Offer and, in the worst case, selling privileges.