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Part 6 of 7 · Gift Aid claimer series ~5 min read

What the gift aid claimer costs

The model runs once per declaration, not once per donation. A charity collecting a few hundred new declarations a year is a few cents; the donation file is parsed. Six charities is a small federation or a diocese sharing one deployment. Here is where each cent goes.

Key takeaways

  • About $2 a month at 6 charities. Roughly $3 at 20 charities.
  • One Bedrock read per charity is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Nothing runs per donation, so a charity that doubles its income does not double its bill.
  • 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 gift aid claimer at three volumesA stacked bar chart with three bars, one per volume tier: 1 charity totalling about $1, 6 charities totalling about $2, and 20 charities totalling about $3. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per charity, 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~$1.371 charity~$1.846 charities~$3.1420 charitiesBedrock — one read per charitySES — 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 charity — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 6 charitiesHow it scales
Bedrock read$0.54Linear. One call per charity, roughly 1,800 in and 200 out tokens.
SES$0.00Linear. About 3 messages per charity.
DynamoDB + S3$0.33Storage 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 new declaration, against a mid-tier model. A declaration is a short document with five fields and a date, and the date is the only one that is ever ambiguous. This is not a place where a frontier model earns its price, and the volume is a few hundred a year rather than a few thousand a day.

The three ways this bill surprises you

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

  • Running a model over the donation export. It is fixed-header CSV and it is the largest file here. Parse it; a model would cost more than the Gift Aid on it.
  • Re-reading declarations to answer a routing question. Extract once into a coverage window and query the window.
  • Storing declaration scans at full resolution for seven years. Keep the original until the claim is filed, then a downsized copy with the evidence.

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. 1 charity, about $1. 6 charities, about $2. 20 charities, 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~$11 charity~$16 charities~$220 charities~$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.

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