Lob

Lob occupies a unique niche: it's the REST API for physical mail. Core services are Print & Mail (postcards, letters, checks, self-mailers) and Address Verification (USPS CASS-certified standardization and deliverability checking). For agents, relevant capabilities are: programmatic check issuance for fintech flows, physical letter delivery for regulated communications requiring paper trails, and address verification for e-commerce or logistics pipelines. Test mode allows development without incurring print costs. Clean resource model, strong idempotency support, and official SDKs in 6 languages. Confidence is docs-derived.

RESTful API at api.lob.com/v1. Resources: postcards, letters, self_mailers, checks, addresses. Idempotency key support on all POST operations. Address verification via /us_verifications and /intl_verifications endpoints. HTML or PDF template support for mail content with custom merge variables. Webhook events for full mail lifecycle (created, in_transit, in_local_area, processed_for_delivery, re-routed, returned_to_sender). List endpoints support filtering by date, metadata, and send date.

Structured error responses: HTTP status with JSON body containing error.code and error.message. Documented error codes for invalid addresses, template rendering failures, and print failures. Test mode provides full error simulation without real-world consequences. Rate limits documented per endpoint (address verification has higher limits than mail creation). Mail cancellation window available (30 minutes after creation for some mail classes).

HTTP Basic auth using the API key as the username (password field empty) — standard and widely supported by HTTP client libraries. Separate test vs. live API keys. Test keys trigger print simulation without actual mail delivery or charges. Publishable keys available for client-side address verification. Webhook HMAC-SHA256 signatures for delivery event verification. No fine-grained per-resource key scoping; keys have full account access.

docs.lob.com is comprehensive and well-organized: quickstart, API reference, code examples in 6 languages, and a dedicated address verification guide. Official SDKs for Node.js, Python, Ruby, PHP, Java, and Elixir. Test mode is built into every API call — no separate sandbox environment to configure. Error messages are descriptive; address verification responses include standardized form, deliverability score, and vacancy info.