Why what-if scenario planning matters for FBA
Every purchase order is a bet on future demand. A single-point demand forecast gives you one number, but it hides the range of outcomes. What-if scenario planning forces you to think about what happens if demand comes in 30% above forecast (you ordered too few and stockout) or 30% below (you’re sitting on excess inventory paying storage fees).
For FBA sellers placing $10,000-$50,000 POs with 60-90 day lead times, the cost of being wrong is significant in either direction. A stockout on a top SKU can lose $500-$2,000/day in revenue plus organic rank decay. Excess inventory on a slow seller can cost $0.50-$2.00 per unit per month in aged inventory surcharges. Scenario planning puts dollar amounts on both risks so you can make an informed decision.
The framework is straightforward: define 3 scenarios, estimate the probability of each, calculate the financial outcome of your order quantity under each scenario, then pick the option with the best expected value.
The decision framework
Example: Q4 order for a $48 kitchen knife set
You sell a kitchen knife set at $48 ASP with $14 landed cost. Your seasonality index says Q4 sells 1.8x your baseline, but you’re uncertain about this year’s demand. You’re placing your Q4 PO with a 78-day lead time and need to decide: order 2,000, 2,800, or 3,600 units?
Three scenarios for 90-day Q4 demand:
| Scenario | Demand | Probability |
|---|---|---|
| Weak Q4 (economy slows) | 2,100 units | 25% |
| Expected Q4 | 2,800 units | 50% |
| Strong Q4 (viral boost) | 3,500 units | 25% |
Assume: Margin per unit sold = $48 – $14 = $34. Excess storage + disposal cost = $6/unit. Lost margin per stockout unit = $34.
Option A: Order 2,000 units
| Scenario | Revenue | COGS | Stockout cost | Profit |
|---|---|---|---|---|
| Weak (2,100) | $96,000 | $28,000 | $3,400 | $64,600 |
| Expected (2,800) | $96,000 | $28,000 | $27,200 | $40,800 |
| Strong (3,500) | $96,000 | $28,000 | $51,000 | $17,000 |
EV(2,000): 0.25 × $64,600 + 0.50 × $40,800 + 0.25 × $17,000 = $40,800
Option B: Order 2,800 units
EV(2,800): 0.25 × ($95,200 – $39,200 – $4,200) + 0.50 × ($134,400 – $39,200) + 0.25 × ($134,400 – $39,200 – $23,800)
= 0.25 × $51,800 + 0.50 × $95,200 + 0.25 × $71,400 = $78,400
Option C: Order 3,600 units
EV(3,600): 0.25 × ($100,800 – $50,400 – $9,000) + 0.50 × ($134,400 – $50,400 – $4,800) + 0.25 × ($168,000 – $50,400)
= 0.25 × $41,400 + 0.50 × $79,200 + 0.25 × $117,600 = $79,350
The math says order 3,600 units. But the difference between 2,800 and 3,600 is only $950 in expected value, while the downside risk (weak Q4 scenario) is $10,400 worse with 3,600 than 2,800. A capital-constrained seller might prefer 2,800 units to limit downside exposure. That’s the decision scenario planning enables: you see the tradeoffs in dollars, not guesses.
FBA-specific scenario variables
Beyond demand uncertainty, FBA sellers should model these variables in their what-if scenario planning: Amazon storage fee changes (Q4 surcharges can double monthly storage costs), restock limit changes (Amazon can cut your capacity mid-cycle), lead time variability (what if the shipment arrives 10 days late and misses the selling window?), and competitive entry (a new competitor launches and takes 15% of your market share).
Each variable adds a branch to your scenario tree. You don’t need to model every combination. Focus on the 2-3 variables with the biggest dollar impact on the specific PO you’re placing. Inventory planning tools can automate scenario modeling across your entire catalog.
Common mistakes
- Only modeling the expected case. A single-point forecast is not scenario planning. You need at least 3 scenarios (pessimistic, expected, optimistic) with probabilities that sum to 100%.
- Assigning equal probability to every scenario. If last year’s Q4 was strong and the macro environment is similar, weighting the “expected” case at 50% and the extremes at 25% each is more realistic than a flat 33/33/33 split.
- Ignoring the cost of being wrong in each direction. Stockout cost and excess inventory cost are rarely symmetric. If stockout cost per unit ($34 in lost margin plus rank damage) is much higher than excess cost ($6 in storage), the math tilts toward ordering more. Run the numbers to see which direction.