Skip to content
allanninal.dev/build
Collaborate

Part 6 of 7 · Card fee auditor series ~5 min read

What the card fee auditor costs

The model runs once per statement, which is once per merchant account per month. A fourteen-page itemised PDF is the most expensive read in this series and it is still four cents. Nine accounts is a small group of sites or a franchisee. Here is where each cent goes.

Key takeaways

  • About $2 a month at 9 accounts. Roughly $3 at 30 accounts.
  • One Bedrock read per merchant account is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Transaction files are parsed without a model at all, so the cost scales with statements rather than with sales volume.
  • The duplicate test runs before the read, so resends are free.
  • The three real risks: a retry loop, storage nobody expires, and a bigger model than the job needs.

The bill at three volumes

These are US East prices at the time of writing, at three volumes that bracket most small businesses. Find the bar closest to your own and read across.

Monthly cost of the card fee auditor at three volumesA stacked bar chart with three bars, one per volume tier: 2 accounts totalling about $1, 9 accounts totalling about $2, and 30 accounts totalling about $3. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per merchant account, in teal. Then SES for the messages, in pink. Then S3 and DynamoDB storage in green. Then a fixed orange band for Secrets Manager and AWS Budgets, which is eighty-six cents at every volume. Then a grey band for Lambda, SQS and CloudWatch. A note says the read is the only bar that grows with the business and the orange band never moves.$0$1$2$3$4~$1.342 accounts~$1.679 accounts~$2.6130 accountsBedrock — one read per merchant accountSES — asks, results, receiptsS3 + DynamoDBFixed — Secrets Manager, BudgetsLambda, SQS, CloudWatchThe read is the only bar that grows with the business. The orange fixed band never moves.
Fig 1. The monthly bill at three volumes. The teal band — one model read per merchant account — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 9 accountsHow it scales
Bedrock read$0.38Linear. One call per merchant account, roughly 1,800 in and 200 out tokens.
SES$0.01Linear. About 4 messages per merchant account.
DynamoDB + S3$0.31Storage grows with what you retain, not with throughput.
Lambda + SQS$0.12Linear, and effectively free at this scale.
CloudWatch$0.16Flat, if you set retention. Unbounded if you do not.
Secrets Manager$0.40Flat. One secret, $0.40 a month.
AWS Budgets$0.46Flat. Two actions, so you find out before the bill does.

One call per statement against a capable model, because merchant statements are the worst documents in this entire series: multi-column, abbreviated, inconsistent month to month, and frequently a scan of a print. This is a place where paying for a better model is straightforwardly correct, and it is still under five cents per account per month.

The three ways this bill surprises you

Every one of these has happened to somebody, and all three are cheap to prevent.

  • Sending transaction files through a model. They are CSV with a fixed header. Parse them; a model here would cost more than the fees you are auditing.
  • Re-reading the statement to answer a new question. Extract once into fees and query the fees. The PDF is the source, not the working set.
  • Storing card numbers because the export contains them. Drop the PAN at ingest. You need the card type and the entry mode, and nothing that puts you in scope.

What it costs when nothing happens

This matters more than the headline number for a seasonal business. In a month with nothing to process the bill is the fixed band: Secrets Manager at forty cents, AWS Budgets at forty-six, and a few cents of storage. Call it a dollar. There is no instance to stop and nothing to remember to turn off.

The monthly bill at four volumes plus one failure modeA horizontal row of five boxes. Quiet month, about one dollar. 2 accounts, about $1. 9 accounts, about $2. 30 accounts, about $3. And one bad retry loop, about two hundred dollars. A note says four of these are the design working and the fifth is a missing dead-letter queue.THE BILL, AT A GLANCEQuiet month~$12 accounts~$19 accounts~$230 accounts~$3One bad loop~$200Four of these are the design working. The fifth is a missing dead-letter queue.
Fig 2. The bill at a glance, including the one that is not a volume at all. A retry loop with no dead-letter queue costs more than every legitimate use of the system put together.
  • Management
  • Analytics
  • Front-end & mobile

Set these on day one

  • A dead-letter queue on every SQS queue, with a maximum receive count of three.
  • Thirty-day retention on every CloudWatch log group. There is no default that is safe.
  • An S3 lifecycle rule on the object prefix, tiering at 90 days and expiring at your actual record-keeping horizon.
  • Two AWS Budgets actions — one that emails at half your expected spend, one at double it. The second is how you find out about a loop in an hour instead of a month.
  • Provisioned concurrency: none. Nothing here is latency-sensitive enough to justify paying for a warm function.

Next: the same system drawn for engineers — service names, resource identifiers, IAM scopes, table schemas and the model id.

All posts