Five engineering decisions to make before automating your billing system

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Automating billing goes beyond sending invoices automatically — it requires ensuring the right amount is charged to the right person exactly once, with reliable confirmation. Developers must validate webhook signatures using HMAC hashes to prevent forged payment notifications from triggering false confirmations. Idempotency keys based on unique gateway transaction IDs are essential to avoid processing the same payment event multiple times due to network retries. A periodic reconciliation job that queries the gateway's API for pending charges acts as a safety net when webhooks are missed due to server downtime or deployment issues. Sensitive card data should never be stored directly; tokenization offered by payment gateways keeps systems out of costly PCI-DSS compliance scope, and dunning sequences must be capped and spaced to avoid crossing into harassment territory.

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