Why Are My Amazon FBA Fees Different Every Month?
Your FBA fees move month to month because several of them are time- and condition-dependent, not fixed per unit. Fulfillment fees can shift when a product crosses a size or weight tier, storage fees rise sharply in Q4, aged inventory triggers long-term surcharges, and Amazon periodically updates the fee schedule itself.
The short answer
The per-unit referral and fulfillment fees feel fixed, but the storage and surcharge layer is where the variation lives. Storage fees spike October-December, aged inventory accrues surcharges that escalate the longer it sits, products can flip size tiers, and Amazon updates the fee schedule periodically.
1. Monthly storage fees spike in Q4
Storage is charged per cubic foot per month, and the rate increases substantially from October through December to discourage hoarding during peak season. The same inventory costs more to store in November than in May, with no change on your end.
2. Aged / long-term storage surcharges
Inventory sitting beyond a threshold (commonly past several months) accrues an additional aged-inventory surcharge that escalates the longer it sits. A slow-moving SKU's storage cost climbs every month until it sells or you remove it.
3. Size/weight tier boundaries
Fulfillment fees are bucketed by size tier and weight band. A product near a boundary can flip tiers if Amazon re-measures it or your packaging changes slightly — and the fee jumps to the next bracket. Dimensional weight (volume-based) can also govern the charge instead of actual weight for bulky-but-light items.
4. Low-inventory and placement fees
Amazon has added fees tied to inventory levels and inbound placement that vary with how you send stock in and how thin your supply runs. These fluctuate with your operations, not a fixed rate.
5. Fee schedule updates
Amazon revises published rates periodically. A month-over-month change can simply be a rate update that took effect mid-period.
Worked illustration: same SKU, two months
May: 15% referral -$3.00, standard fulfillment -$3.86, monthly storage -$0.18, no aged surcharge. Total fees: $7.04.
November: same referral and fulfillment, monthly storage at peak Q4 rate -$0.63, aged-inventory surcharge -$0.40. Total fees: $7.89.
Identical product, $0.85 more in fees in November — driven entirely by storage seasonality and aging.
How to reconcile and reduce variance
Pull the monthly Storage Fees report and the Aged Inventory report separately from per-unit fulfillment. Identify SKUs accruing aged-inventory charges and clear or remove them before surcharges escalate. Check whether a SKU recently changed size tier (Manage Inventory → fee preview). Plan inventory so you're not over-stocked going into Q4 storage rates.
Frequently asked questions
- Why did my FBA fees go up?
- Most often it's Q4 storage rate increases, an aged-inventory surcharge kicking in, a size-tier re-measurement, or an Amazon fee-schedule update. Pull storage and aged-inventory reports separately to isolate which.
- Do FBA storage fees increase in Q4?
- Yes — Amazon charges a substantially higher per-cubic-foot rate from October through December to discourage hoarding during peak season. The same inventory costs more to store in November than in May.
- What is the FBA aged inventory surcharge?
- An extra fee on units that have been in FBA fulfillment centers beyond a threshold (commonly past several months). The surcharge escalates the longer stock sits, on top of normal monthly storage.
- How are FBA fulfillment fees calculated?
- By size tier and weight band, with dimensional weight (volume) sometimes governing the charge for bulky-but-light items. A small change in measured dimensions can flip a SKU into the next tier.
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