Price a footprint, or price a budget

Name exactly one of carbon_kg (what does this footprint cost) or amount_usd (what does this budget buy). Quoted live on every call and never from a cache. The price is ALL-IN: cost_usd is the one number, and no fee, margin, cost-basis or per-tonne rate is returned beside it. A reverse quote answers whole kilograms, ceiling-rounded to at least one, so cost_usd can exceed amount_usd by that rounding. type picks the supplier as well as the cart: offset is priced on the CNaught products, removal on the Stripe Climate products.