FBA or FBM? Decide Product by Product With the Full Per-Unit Cost

Many sellers choose FBA or FBM once and apply the answer to every product. It is a convenient rule and an expensive one, because the cheaper method changes with the size of the item, how quickly it sells, how often it comes back and how much the Prime badge adds for that product. The fair comparison is the full cost to fulfill one sold unit under each method, followed by an honest estimate of what Prime eligibility is worth. This article sets out both steps and works through a hypothetical small item and a hypothetical bulky one.

Fulfillment is a per-unit cost, so compare it per unit

Asking which method is cheaper in general has no answer. Ask instead what each method adds to the cost of a unit that has just sold. Some costs appear in both columns under different names, some appear in only one, and a few are easy to forget.

Notice the sneaky ones: inbound freight and prep for FBA, and your own time for FBM. A comparison that includes only the fulfillment fee on one side and only postage on the other leaves out costs that can decide the result.

Two hypothetical products, side by side

Filling in the FBM column takes more work than the FBA column, because Amazon publishes its rates and you have to build your own. Time how long a person takes to pick, pack and label an order, multiply by the hourly cost of that person, and add the packaging materials. Use real carrier quotes for the typical box size and zone, not the cheapest rate you have ever seen. Then add a returns allowance based on what you actually get back, and the minutes your team spends on customer messages.

Product S is a small, light item priced at $22 that sells quickly. Product B is bulky, priced at $90 and sells slowly. All figures below are invented round numbers to show the method.

For the FBM columns, product S breaks down as $1.50 of pick and pack, $0.40 of packaging and $4.80 of postage, which is the $6.70. Product B breaks down as $3.00 of pick and pack, $2.50 of packaging and $9.00 of postage, which is $14.50.

Product S is cheaper on FBA by $3.50 per unit. Its storage cost is tiny because it sells fast, so units do not sit long. Product B reverses the result: FBA costs $4.00 more per unit, mainly because storage is billed on bulk, and the bulky item sits for a long time before it sells. Your own space is cheaper for it.

The slow seller is also the product most likely to run into aged-inventory surcharges, which the table does not even include. The faster a unit turns, the less FBA storage matters to its cost.

Pricing the Prime badge

Cost is half the decision. FBA offers carry the Prime badge, and many shoppers filter for it, so a product may sell more units on FBA than on FBM even when FBA costs more per unit. Fulfillment method is also one of the factors Amazon has described as part of how it chooses the Featured Offer, so an FBM offer can lose the Buy Box to a Prime-eligible competitor. You cannot read the badge's value from a report. You can still calculate how large it has to be.

Say the bulky item costs $30 to buy and land, and the referral fee is assumed at 15% of its $90 price, which is $13.50. That leaves $46.50 per unit before any fulfillment cost. On FBA that leaves $24.00 after fulfillment costs, and on FBM it leaves $28.00. For FBA to match the total profit of FBM, it has to sell enough extra units to close that gap:

$28.00 divided by $24.00 is 1.17, so FBA must lift unit sales by about 17% for the bulky item to come out ahead. Is that plausible? Judge it with your own evidence: sales when you held both options, the competitors' fulfillment mix on the same search results, and how often shoppers see an FBM offer lose the Featured Offer. For product S the question disappears, because FBA is cheaper and carries the badge. Run the same calculation for every product that looks cheaper on FBM and rank them by the lift required. A small required lift suggests staying on FBA, and a large one makes the switch to FBM more convincing.

FBM also carries a risk that the cost table cannot show. Orders you ship yourself count toward your seller performance metrics, so a late shipment or a cancellation lands on your account, and a missed carrier pickup in a busy week is your problem to fix. That risk does not need a number on the sheet, but it should influence how thin a margin you are willing to accept on FBM.

FBM sellers who want the badge can look at the program for merchant-fulfilled Prime. It has delivery-speed and performance requirements that change, so check eligibility and the cost of meeting them before counting on it.

A rule of thumb, and its exceptions

Size and sales speed predict much of the answer, as the grid below shows. It is a starting point for the per-SKU math, not a replacement for it.

Several things can overrule the grid. Fragile items raise damage risk in a long chain of handling. Products near FBA size or weight limits may switch tiers after a packaging change. Seasonal goods build up storage costs that an FBM warehouse may absorb more cheaply. Return-prone products cost less to handle if you can inspect and resell them yourself. Cash flow matters as well, because stock in an Amazon fulfillment center is stock you cannot redirect elsewhere.

Costs also move. Carrier rates, Amazon's fee schedule and your own sales speed all change during the year, and each change can tip a product from one column to the other. A product that is marginally cheaper on FBM today may flip after a fee update or after it starts selling faster. That is why the sheet needs a review date and not just a verdict.

Where to start

• List your products by size tier, monthly units and days of stock on hand.
• Build a per-unit cost sheet for each under both methods, including inbound freight, prep, storage, returns and your own time.
• For products where FBM is cheaper, calculate the sales lift that FBA would need and compare it with your evidence.
• Check restrictions, size limits and capacity limits before you switch anything.
• Revisit the decision each quarter, or when fees, carrier rates or your sales speed change.