Build Ecommerce Unit Economics Before Inventory

Build Ecommerce Unit Economics Before Ordering Inventory

Build ecommerce unit economics from net revenue, complete variable cost, contribution, break-even volume, cash timing, and downside. Do not start with list price minus supplier quote. That shortcut excludes the costs most likely to surprise a first inventory decision.

This guide uses educational formulas and illustrative categories. The actual model requires current seller, supplier, product, marketplace, tax, logistics, and professional inputs.

flowchart LR
    A["Net revenue"] --> B["Minus variable cost"]
    B --> C["Contribution per unit"]
    C --> D["Break-even volume"]
    D --> E["Cash cycle"]
    E --> F["Base and downside cases"]
    F --> G["Inventory limit"]

1. Calculate net revenue

Begin with the amount the seller expects to retain before variable cost.

Gross selling price may be reduced by discounts, coupons, refunds, taxes collected for authorities, marketplace adjustments, and other deductions that do not belong to the seller. Record each line rather than hiding them inside one percentage.

Use an expected selling price grounded in the test and market record. A competitor's list price does not prove that buyers will accept the same price from a new offer.

2. Build landed product cost

Landed cost should bring one saleable unit to the point where the fulfillment path can accept it.

The model may need manufacturing, product packaging, labeling, inspection, freight, insurance, duties, brokerage, domestic movement, preparation, loss allowance, defects, and currency effects. The exact set depends on the product and sourcing path.

Separate quoted, observed, and assumed inputs. Record the quote date, quantity, payment terms, Incoterm where relevant, currency, and what the supplier excluded.

3. Add channel and fulfillment cost

Marketplace, payment, fulfillment, storage, inbound placement, return, removal, and optional-service costs can vary by product, marketplace, size, weight, category, inventory age, and service selection.

Amazon's pricing page distinguishes selling-plan fees, referral fees, FBA costs, and optional costs. Its fee-estimation workflow allows a seller to compare Amazon fulfillment with seller fulfillment using a defined product and adjustable inputs.

Use the current account preview for the actual product. A general fee page provides categories and routes. It is not a seller quote.

4. Model returns, damage, and customer service

Returns affect revenue, processing, transport, inspection, resale value, disposal, customer support, and inventory availability.

Do not copy a general category return rate into the expected case without identifying its source and relevance. Use observed test data when available. Until then, run a range and explain the uncertainty.

The downside case should include a worse return or defect outcome than the expected case. If the model fails with a plausible change, the business needs a better product, price, process, test, or commitment size.

5. Add acquisition cost

Acquisition includes media, creative production, samples, discounts, affiliates, agencies, software, and attribution error.

For a paid channel, model clicks, conversion, cost per click, and resulting acquisition cost as separate assumptions. That makes the failure visible. A small conversion change can alter contribution even when the advertising price stays constant.

Do not assume every repeat purchase is free. Retention may require service, messaging, incentives, inventory availability, and another reason to choose the product.

6. Calculate contribution

Contribution per unit equals net revenue per unit minus variable cost per unit.

Contribution margin ratio equals contribution per unit divided by net revenue per unit. The SBA break-even guide uses sale price, variable cost, fixed cost, and contribution to calculate a basic break-even point.

The calculation should be visible:

A negative or fragile contribution is not repaired by selling more units. Volume can increase the loss.

7. Model fixed operating cost

Fixed and step costs may include software, insurance, professional services, storage commitments, salaries, subscriptions, testing, compliance, creative systems, and administration.

Decide which costs belong to the product decision and which belong to the wider company. Show the allocation method. Do not bury a desired result inside an arbitrary overhead assumption.

8. Map the cash cycle

Profitability and liquidity are different.

Record supplier deposit, final payment, inspection, freight, transit, customs, receiving, launch spend, inventory days, marketplace payout timing, refunds, reserves, and the point at which cash becomes usable again.

The Amazon FBA inventory record describes excess, aged, and stranded inventory as operational conditions sellers may need to manage. Slow inventory can produce storage and working-capital consequences even when the expected per-unit contribution remains positive.

9. Run base, downside, and break-even cases

The base case should use the best current evidence, not the most attractive inputs. The downside case should combine plausible adverse changes in price, conversion, acquisition, returns, lead time, freight, defects, and sell-through.

The break-even case should show the price, volume, acquisition cost, or return rate at which contribution or cash capacity fails. Then identify which input is both important and weakly supported.

10. Convert the model into an inventory limit

Do not let the supplier's minimum order become the business's approved order.

Set a maximum cash exposure, minimum post-payment runway, acceptable loss, evidence threshold, test quantity, review date, and exit plan. Include the cost of discounting, removal, liquidation, or disposal when demand is weaker than expected.

The result should be a range and a decision, not a decorative spreadsheet.

Continue with [[Make an Inventory Commitment Decision]] to compare test, wait, reject, and commit. [[How to Evaluate an Ecommerce Product Before Committing Inventory]] supplies the wider evidence record.

This guide was developed with AI assistance from E011 and the linked Amazon and SBA sources. It is not financial, accounting, tax, legal, logistics, or investment advice. Publication remains unauthorized pending financial, accounting, tax, platform, accessibility, and founder review.