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Q4 storage costs 2.8× normal. Your margin has to survive it.
A $29.99 product with $8 landed cost nets $12.09 a unit after Amazon's referral and fulfillment fees — until 500 units sit in a Q4 warehouse at $2.40 per cubic foot. This calculator runs per-unit margin against the holiday storage bill so the shipment is priced before it ships.
FBA sellers use it before the shipment: $12.09/unit margin meets $360/mo of Q4 storage at 2.8× rates — send what sells by Christmas, not what fits the container.
The fee stack per unit: referral fee (15% of sale price for most categories — 8% for electronics, tiered for a few others), fulfillment fee by size tier (roughly $3.22–$6+ for standard-size items under 3 lb, more for oversize), and monthly storage — which is where Q4 bites: roughly $2.40 per cubic foot October–December versus $0.87 January–September, a 2.8× multiplier. Add the aged-inventory surcharge for anything sitting 181+ days — inventory that misses the Christmas window gets taxed for staying. The calculator's defaults reflect current published rates; always pull your exact fulfillment fee from the Seller Central revenue calculator because size-tier boundaries move every year.
The storage math inverts the instinct to max out: every unit past your sell-through forecast costs $2.40/cu ft per month and risks the aged-inventory surcharge in January. Work backward from last year's Q4 velocity (or category benchmarks for a launch), forecast weekly sales, and size the shipment to sell through by December 25th plus a 10–15% buffer — not to maximize stock. A 500-unit shipment of a 0.3 cu ft item pays $360/month at Q4 rates; the same shipment sitting in February costs $131/month but the surcharge clock is running. Send in waves if your supplier lead time allows — a second shipment in early November beats 500 units aging since September.
The veteran rule is a minimum 30% margin after ALL Amazon fees at the intended sale price — and 40%+ if the product is seasonal, because Q4 storage and post-holiday returns erode the headline number. Below 25% margin, one fee change or a forced price match erases the product. The margin test that matters: run the calculator at your planned price, then at 15% off (the Q4 coupon reality), then with 2 extra months of storage — a product that stays profitable through all three scenarios is a real product. Private-label launches should also reserve 10% of revenue for PPC in the first 90 days; the margin on paper assumes organic sales that do not exist yet.