Source priority
We use the marketplace or payment provider's current official fee policy, legal terms, or help center as the primary source. Third-party articles can help identify a change, but they do not become a calculator default until the platform's own material supports it.
Separate every fee base
Platforms do not always apply every percentage to the same amount. Commission may use item price while processing uses item price, buyer shipping, and tax. The engine calculates those components separately before combining them.
Separate payout from profit
Gross payout is the amount credited from the transaction before fees. Net payout subtracts modeled platform and processing fees. Profit then subtracts inventory cost, seller-paid shipping, packaging, and other direct costs entered by the user. Margin divides profit by seller revenue. ROI divides profit by direct cost.
Reverse pricing
Marketplace calculators can solve the sale price needed for a target dollar profit. The engine repeatedly evaluates the complete fee formula until it finds the lowest price that reaches the target under the entered assumptions. This handles flat charges and tier thresholds that a single shortcut formula can miss.
Quality control
Every calculator receives unit tests for representative transactions, edge cases, thresholds, and input validation. Browser tests cover live recalculation, mobile overflow, navigation, accessibility basics, and failed network resources. Production checks verify canonical URLs, sitemap coverage, structured data, policy pages, and LLM-readable files.
Update policy
Rate-sensitive pages show the date their source was reviewed. Automated editorial monitoring can surface fee changes, but an official source and formula test are required before publication. Actual account statements remain the final authority.