payments

Payments engineering is mostly failure engineering. This lesson covers the decline taxonomy and what a merchant may do about each kind, how strong customer authentication and 3-D Secure actually work including the exemptions with their 0.13 and 0.06 percent fraud thresholds, who eats a fraudulent transaction and when that liability shifts, and the chargeback machine from dispute to representment.

Every card fee argument eventually reaches interchange: the fee the merchant's side pays to the cardholder's bank on every transaction. This lesson decomposes the merchant discount rate, explains why rewards cards exist and who really funds them, covers the EU caps of 0.2 and 0.3 percent and their loopholes, and shows why the same purchase costs a merchant triple on a corporate card.

The tap is instant; the money is not. A card payment runs on three separate clocks: authorization in about a second, clearing in overnight batches, settlement as netted bank transfers. This lesson walks the message flow, explains holds and why they linger, shows why refunds are slow when charges are fast, and follows one transaction through all three phases.

A card payment is a trust machine between four parties who have mostly never met: cardholder, issuer, merchant, acquirer, with a network in the middle that routes messages and sets the rules. This lesson maps who does what, who carries which risk, what a PSP actually is, and the surprising answer to whose money pays the merchant on the day you tap.

The 2025 GENIUS Act turned stablecoins from an unregulated experiment into a defined instrument with reserve rules, permitted issuers, monthly attestations, and a ban on paying holders yield. This lesson covers what the law requires, why the yield ban shapes the whole business model, and the risks the rules do not remove.

A stablecoin is a claim on an issuer that trades at a dollar because you can redeem it for a dollar. This lesson builds the mechanism: the mint-and-redeem arbitrage that enforces the peg, why reserve composition is the whole game, and what the USDC depeg and the Terra collapse each proved about how pegs actually break.

Scoped tokens and signed mandates solve authorization, not everything. This lesson covers the unsettled parts: who is liable when an agent buys wrong, why merchants and agents have opposing incentives, prompt injection as a new attack surface at checkout, agent-to-agent buying, and why the market forecasts disagree by an order of magnitude.

How do you let software spend your money without handing it your card? This lesson covers the mechanism behind agentic checkout: scoped single-use credentials like the Shared Payment Token, Google's AP2 intent and cart mandates as signed verifiable credentials, agent identification, and the guardrails that bound what an agent may buy.

You cannot understand agentic checkout without understanding checkout. This lesson builds the payment foundation: the five parties in every card transaction, the authorization-capture-clearing-settlement path, who really pays interchange, why card-not-present fraud shapes everything, and the tokenization idea that agent payments are built on.

Western phones hold a dozen apps for a dozen jobs. In China, one app is the messaging, the payments, the taxi, the bill, the government form, and the shop. Learn how the super-app model works, why mini-programs made bundling possible, why a market that skipped credit cards produced a different shape of internet, and what the design actually trades away.