Herminox is six free Amazon calculators for sellers and buyers. Everything runs in your browser — no signup, no email gate, inputs never leave your device. Sellers get FBA profit (2026 fees + fuel), ACoS/TACoS breakeven with max CPC, and inventory recovery: units to get PO cash back after fees. Buyers get unit price, cost per use, and a return tracker with holiday windows and locked capital. Coaches can embed widgets. Independent — not affiliated with Amazon. Confirm fees with Seller Central.