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Part 6 of 7 · Late payment interest claimer series ~5 min read

What the late payment interest claimer costs

The model reads agreements, and a business signs those a few times a year per customer at most. Everything that happens per invoice, per payment and per night is arithmetic against a table. Here is where each cent goes.

Key takeaways

  • About $4 a month at 200 customers. Roughly $14 at 800 customers.
  • One Bedrock read per agreement is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The bill tracks customers, not invoices. Ten thousand invoices a month cost the same as one hundred.
  • 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 late payment interest claimer at three volumesA stacked bar chart with three bars, one per volume tier: 40 customers totalling about $2, 200 customers totalling about $4, and 800 customers totalling about $14. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per agreement, 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$5$10$15$20~$1.8740 customers~$4.43200 customers~$14800 customersBedrock — one read per agreementSES — 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 agreement — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 200 customersHow it scales
Bedrock read$2.80Linear. One call per agreement, roughly 1,800 in and 200 out tokens.
SES$0.16Linear. About 3 messages per agreement.
DynamoDB + S3$0.34Storage 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.

Roughly one model call per customer agreement version, against a mid-tier model. The extraction is narrow: the payment terms, any verification period, any contractual late payment remedy, and the clause each came from. Length is not difficulty, and a frontier model finds the same four clauses for several times the price.

The three ways this bill surprises you

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

  • Re-reading every agreement on every nightly run. Terms change when somebody signs something. Extract once per version and re-read on change.
  • Recomputing every open debt every night. Store the daily amount and multiply by days. A nightly full recalculation over six years of history is the one way to make this expensive.
  • Keeping every accounting export forever. Keep the derived debt record, which is small and has to last six years, and expire the raw exports once reconciled.

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. 40 customers, about $2. 200 customers, about $4. 800 customers, about $14. 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~$140 customers~$2200 customers~$4800 customers~$14One 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.

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