Card fee auditor
A small system for anybody who takes card payments: a monthly statement read into individual fees, an effective rate computed per transaction rather than quoted, the downgraded transactions identified by why they downgraded, and a comparison that survives contact with a salesperson. Seven posts on the same system, one diagram at a time, with a cost breakdown and an engineering reference at the end.
- 01
A card fee auditor on AWS for a few dollars a month
The whole system on one page — the statement, the transactions, the effective rate and the downgrades — and why the rate you were quoted was never going to be the rate you pay.
- 02
How a statement becomes forty itemised fees
Extracting itemised fees from a hostile document, classifying each as interchange, scheme or margin, and treating unmatched fees as questions.
- 03
The effective rate, and why it is not what you were quoted
Computing an effective rate that survives scrutiny, what has to be included, and separating the negotiable part from the pass-through.
- 04
Why some transactions cost four times more
Joining fees to transactions, identifying downgrades by cause, and separating the causes you can change from the customers you cannot.
- 05
Comparing two providers without being lied to
Making two offers comparable, why interchange-plus-plus is more honest than blended, and the contract fields that matter more than the rate.
- 06
What the card fee auditor costs
About $2 a month. One Bedrock read per merchant account is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the card fee auditor architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is an effective rate?
- Total fees divided by total card turnover, as a percentage. It is the only number that compares two providers honestly, and it is never the number on the front of the quote.
- Why is my effective rate higher than the rate I was quoted?
- Because the quoted rate covers one card type presented one way. Corporate cards, overseas cards and keyed-in transactions all cost more, and the quote is the floor rather than the average.
- What is a downgrade?
- A transaction that failed to qualify for the rate you expected and was charged at a higher one — usually because data was missing, the card was corporate, or authorisation and settlement did not match.
- Can I actually reduce these fees?
- Some of them. Interchange is set by the card schemes and no provider can discount it. The acquirer margin is negotiable, and the downgrades are usually fixable at the terminal.
- What does it cost to run?
- A couple of dollars a month. See part six.