# Charge \[One-time payments] The RedotPay `charge` intent is a one-time payment flow using MPP's `402 -> Credential -> 200` pattern. ## Method / intent * `method="redotpay"` * `intent="charge"` ## Install ```bash npm i @redotpay/mpp mppx ``` ## Server integration Use a single import to get both `Mppx` and the RedotPay method factory: ```ts import * as mppx from 'mppx' import { Mppx, charge } from '@redotpay/mpp/server' const consumed = new Set() const payment = Mppx.create({ methods: [ charge({ consumeReference: async ({ reference }) => { if (consumed.has(reference)) return false consumed.add(reference) return true }, verifyBalance: async ({ rdt }) => Boolean(rdt), verifyCrypto: async ({ hash }) => Boolean(hash), }), ], realm: process.env.MPP_REALM, secretKey: process.env.MPP_SECRET_KEY!, }) export async function handler(req: Request) { const authorization = req.headers.get('Authorization') if (!authorization) { const challenge = await payment.challenge.redotpay.charge({ amount: '1.00', decimal: 0, currency: 'usd', methodDetails: { balance: {} }, } as any) return new Response(null, { status: 402, headers: { 'WWW-Authenticate': mppx.Challenge.serialize(challenge) }, }) } const receipt = await payment.verifyCredential(authorization) return new Response(JSON.stringify({ ok: true, receipt }), { status: 200, headers: { 'Content-Type': 'application/json' }, }) } ``` ## Client integration The client handles `402 -> Credential -> retry`: ```ts import * as mppx from 'mppx' import { charge } from '@redotpay/mpp/client' export async function fetchWithAutoPay(url: string) { const first = await fetch(url) if (first.status !== 402) return first const wwwAuthenticate = first.headers.get('www-authenticate') if (!wwwAuthenticate) throw new Error('missing WWW-Authenticate') const method = charge({ payload: { type: 'balance', rdt: 'rdt_xxx' } }) const challenge = mppx.Challenge.deserialize(wwwAuthenticate, { methods: [method] }) const authorization = await method.createCredential({ challenge }) return fetch(url, { headers: { Authorization: authorization } }) } ``` ## Request shape (decoded Challenge request) * `amount`: string * `decimal`: number (default 0) * `currency`: string (lowercased) * `methodDetails.balance` and/or `methodDetails.crypto[]` ## Credential payload (decoded Credential payload) * Balance rail: `{ type: "balance", rdt: string, externalId?: string }` * Crypto rail: `{ type: "crypto", chainId: number, currency: string, hash: string, externalId?: string }`