FBA vs FBM: A Net-Profit Decision Tree

When Amazon FBA fees rise — and they rose twice between 2024 and 2026 — Fulfilled by Merchant looks attractive. It is sometimes the right move. It is more often a margin trap dressed up as a margin save. The decision requires comparing not just the headline fee difference but every downstream cost and conversion impact.

The true cost of FBA

FBA pick and pack fees in 2026 range from $3.22 for a small standard item (under 4oz) to $6.92 for a large standard item (between 2–3 lbs). Oversize items start at $9.73 and increase steeply with weight. On top of pick and pack, you pay monthly storage fees: $0.78 per cubic foot for standard size from January through September, $2.40 per cubic foot from October through December, and long-term storage fees of $6.90 per cubic foot for any inventory over 365 days.

The accurate FBA cost for any SKU is: pick and pack fee + monthly storage allocation (unit cubic feet × storage rate × average months held) + inbound shipping to FBA + prep fees if applicable + long-term storage reserve if sell-through is slow. Most sellers calculate only the pick and pack fee and wonder why their margins disappear.

The true cost of FBM

FBM looks cheap because there is no visible fee line item from Amazon. The real costs are: your outbound shipping cost (carrier + packaging materials), your inbound receiving cost if you use a 3PL, the labour cost to pick, pack, and ship each order, customer service time for shipping inquiries and tracking updates, and the cost of managing returns independently.

A seller doing FBM in-house should fully load their labour cost at a minimum of $15 per hour and allocate realistic time per order (typically 4–8 minutes for pick, pack, label, and dispatch). At $15/hour and 6 minutes per order, direct labour alone adds $1.50 per unit before any materials cost. At volume, a 3PL will outperform in-house FBM economics once you exceed roughly 200 orders per month.

The conversion premium of Prime

This is the number FBM calculations consistently undercount. Prime eligibility — available to FBA sellers automatically and to FBM sellers only via Seller Fulfilled Prime (which has strict performance requirements) — lifts conversion rates by an estimated 30–50% on competitive listings. On a listing converting at 12% FBM, FBA might convert at 15–17%.

That conversion lift compounds. Higher conversion improves organic rank. Better organic rank reduces the ad spend required to maintain visibility. Lower ad spend per unit improves net profit. The Prime effect is not just a direct conversion improvement; it is a multiplier on the efficiency of every marketing dollar you spend on that listing. Any FBA vs FBM comparison that ignores conversion impact is modelling the wrong question.

When FBM wins

FBM wins when your unit is oversize or heavy — the FBA pick fee on a 5lb+ item is substantial and often exceeds what a regional carrier charges for the same shipment. FBM also wins when your sell-through is slow enough to incur significant storage fees, when you have a 3PL with sub-$3 all-in pick costs and fast SLAs, for hand-made or custom items where FBA prep is impractical, and for hazmat products that Amazon is restrictive about storing.

FBM is also worth considering for products with very high ASPs where the incremental Prime conversion lift is smaller in absolute dollar terms and where the customer profile (typically more considered purchasers) does not weight Prime as heavily in their purchase decision.

When FBA wins

FBA wins for small, light, fast-moving SKUs. The economics are clearest when pick and pack fees are under $4, the product moves at 30+ units per month (keeping storage costs per unit low), and the category is competitive enough that Prime visibility materially affects rank and conversion.

FBA is rarely the cheapest fulfillment, but it is often the highest-net.

FBA is also the operationally simpler choice at scale. Customer service, returns processing, and SLA management all transfer to Amazon. The hidden cost of FBM is operational complexity, which compounds as volume grows. Most sellers underestimate how expensive their time is until they try to hire a replacement.

The hybrid model

The most sophisticated Amazon operators run a hybrid: FBA for their top-selling, lightweight, high-margin SKUs and FBM (via a 3PL) for slow-moving, heavy, or oversize inventory. This captures the Prime conversion premium where it matters most while avoiding the FBA storage fees that punish low-velocity products.

Run both scenarios for every SKU in the NetSellerProfit calculator. Add a Prime conversion lift assumption of 1.35x for non-commodity items (lower for commodity categories where buyers are primarily price-driven). Subtract your real customer-service cost allocation for FBM. The number that wins on net profit per unit, accounting for conversion impact, is the right answer.