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FBA vs FBM in the UK: Costs, Control and When to Switch

The honest answer to "FBA or FBM?" is: for most UK sellers, both — FBA for your fast-moving core range, FBM (via your own operation or a 3PL) for everything FBA punishes: slow movers, oversize items, seasonal spikes and anything Amazon's storage fees make uneconomic. Treating it as a one-time either/or decision is how sellers end up either paying Amazon to warehouse stock that doesn't sell, or losing the Buy Box on products that would fly with Prime. Here's how the two models actually compare in the UK, with the switching triggers to watch for.

The mechanics, briefly

FBA (Fulfilled by Amazon): you ship prepped stock into Amazon's UK fulfilment centres. Amazon stores it, picks and ships orders, handles customer service and returns. Your listings get the Prime badge. You pay a fulfilment fee per unit sold, plus monthly storage fees, plus long-term storage surcharges on stock that lingers.

FBM (Fulfilled by Merchant): you list on Amazon but ship orders yourself — from your garage, your warehouse, or a 3PL. You control the stock, the packaging and the carrier. You can win Prime eligibility through Seller Fulfilled Prime, but its performance requirements (including weekend dispatch and tight delivery promises) are demanding enough that most FBM sellers operate without the badge.

Where FBA wins

  • Conversion. The Prime badge measurably lifts conversion rate, and many UK shoppers filter to Prime-only. For competitive categories, this alone can decide the economics.
  • Buy Box weighting. All else equal, FBA offers tend to perform strongly in the Buy Box algorithm because Amazon trusts its own delivery promise.
  • Hands-off scale. A viral spike that would bury a merchant-fulfilled operation is absorbed by Amazon's network without you hiring anyone.
  • Customer service and returns. Amazon handles both, for better or worse. Fewer 2am emails.

Where FBM wins

  • Storage economics. Amazon's monthly storage fees are meaningful, and its long-term surcharges on stock over roughly 6–12 months old are punishing (rates change — check current fee schedules). A 3PL pallet at a few pounds a week is dramatically cheaper for slow-turning stock.
  • Oversize and heavy items. FBA fees scale steeply with size and weight. Bulky products are frequently cheaper to fulfil yourself even after paying a 3PL.
  • Capacity limits. Amazon restricts how much you can send in, especially for newer sellers and around Q4. FBM has no such ceiling — your 3PL just holds more pallets.
  • Control. Your packaging, your inserts (within Amazon's rules), your carrier choice, no commingling risk, and no waiting on a removal order to get your own stock back.
  • Multichannel stock. FBA inventory serves Amazon (Multi-Channel Fulfilment exists but has limits and unbranded constraints). FBM stock at a 3PL can ship for Amazon, your Shopify store, eBay and TikTok Shop from one pool.

Cost comparison: a realistic sketch

Take a 500g product in standard packaging selling at £20. Rough shape of the comparison (Amazon's fees change frequently — verify current rates in Seller Central):

| Cost line | FBA | FBM via 3PL | | --- | --- | --- | | Referral fee (~15%) | £3.00 | £3.00 | | Fulfilment | FBA fee, typically ~£2.50–£3.50 for this size band | £0.95 pick/pack + postage (e.g. ~£2.50–£3.30 tracked 48) | | Storage | Monthly fee + long-term surcharge risk | e.g. £3.25/pallet/week shared across all units on the pallet | | Prep | Still needed before inbound | Included in 3PL handling | | Returns handling | Included, but automatic refund-first culture | Per-unit grading fee, you decide outcomes |

On pure per-order cost the two often land within pennies of each other for standard-size goods. The divergence comes from storage duration (slow stock is far cheaper outside Amazon), size (big items favour FBM), and conversion (the Prime badge favours FBA). Which is why the answer is usually a split, not a side.

When to switch — or split

Triggers that suggest moving a SKU out of FBA:

  1. Sell-through has slowed and long-term storage fees are approaching.
  2. Amazon has cut your capacity allocation and you can't keep the SKU in stock anyway.
  3. The item is oversize and the FBA fee is eating the margin.
  4. You're expanding to Shopify/TikTok and don't want a second stock pool.

Triggers that suggest moving a SKU into FBA:

  1. You're losing the Buy Box or ranking to Prime-badged competitors.
  2. Order volume has outgrown your dispatch capacity and SLAs are slipping.
  3. The product is small, light, fast-turning — FBA's sweet spot.

The hybrid pattern that works for many UK sellers: hold the bulk of stock at a 3PL, drip-feed FBA with 2–4 weeks of cover per shipment, and keep the same SKUs live as FBM offers so a stock-out at Amazon doesn't take the listing dark. You minimise Amazon storage fees, sidestep capacity limits, and never fully lose the sale.

A note on Seller Fulfilled Prime

SFP lets FBM offers carry the Prime badge, but the bar is high: near-perfect on-time dispatch and delivery, weekend operations, and nationwide fast delivery coverage. It's achievable with a capable 3PL running late cut-offs and tracked next-day services, but go in with eyes open — the requirements are Amazon's to change, so check the current SFP programme terms before building a strategy on it.

How Oakmont handles it

Oakmont sits on both sides of this decision. We prep and forward your FBA shipments (from £0.26–£0.50/unit including inspection, bagging and FNSKU labels, with a 24-hour turnaround target), and we fulfil your FBM and multichannel orders at £0.95 per order plus £0.25 per extra unit, with a 2pm cut-off for same-day dispatch. Because both run from the same stock pool with live inventory in the portal, moving a SKU between FBA and FBM is a routing decision, not a logistics project.