At checkout
held€24.00
Her bank says yes and sets the money aside. Nothing has moved.
Module 28 · Card payments
Where the money goes when you pay by card
A card payment looks instant. It is really a hold, a promise and a transfer that night, and every party along the way takes a fee. Here you follow Larkspur’s customers’ money through the tubes, then set a checkout policy for each kind of order.
Read as
The whole idea in one example
Ana’s hinge pack: approved, then paid
That night
paid€23.80
Reaches Larkspur. The rest went in fees to three other parties.
Money comes from a simulated day: Larkspur’s own orders, masked made-up cards and planning-number fees, not anyone’s real prices.
How it works
Four ideas carry the module
1 A hold first
A yes sets money aside; nothing moves
Visa’s guide for US merchants says each approved authorisation reduces the cardholder’s available funds, a hold, and that authorisations a merchant won’t complete must be reversed.
Source, checked 25 October 2026
2 Money moves at night
Each party takes a fee on the way
Under EU Regulation 2015/751 (Article 3), the interchange fee on a consumer debit card payment may be at most 0.2% of its value.
Source, checked 25 October 2026
3 Retries
A lost answer can mean two charges
An IETF Internet-Draft (draft-ietf-httpapi-idempotency-key-header, not yet an RFC) has the client send a unique Idempotency-Key so the server can recognise a retry and not repeat the operation.
Source, checked 25 October 2026
4 Prove it’s you
A challenge stops thieves, and a few buyers
Visa’s dispute guide for merchants (October 2026) says a fully authenticated 3-D Secure payment is protected from fraud disputes; a payment without 3-D Secure is not.
Source, checked 25 October 2026
The story
Larkspur follows the money
A customer pays Larkspur Supply Co. by card; the team follows the money through holds, overnight transfers, fees, retries, fraud checks and chargebacks. You’re on the team that runs the checkout.
- 18
- card orders in Larkspur’s day, the one every chapter runs
- 5
- parties every payment touches, counting the customer and Larkspur
- 13
- days and nights the clock runs, from the orders to the last dispute
- 1
- checkout policy keeps the most in chapter 7
Your path
The chapters
Chapter 1 · done
Four parties and a network
Who does Ana’s card payment pass through, and who says yes?
Chapter 4 · done
Pressed Pay twice
When a payment request gets no answer, how do you retry without charging twice?
Chapter 5 · done
Prove it’s you
How does a bank check it’s really the cardholder, and what does that cost?
Chapter 7 · done
Set Larkspur’s checkout policy
Which checkout settings keep the most money without charging anyone twice?
Who it’s for
- Engineers
- Product managers
- Analysts
- No code needed
Anyone whose product takes card payments, and who wants to know why the money arrives later and smaller, and what a chargeback is. EU rules only; no background needed.