PostGrid

Send real-world mail from FlowRunner through PostGrid. Agents verify and standardize mailing addresses, maintain a reusable contact address book, and send letters and postcards programmatically, with test mode for development.

6 actions API key available

PostGrid website ↗ Platform Documentation ↗ Capability data verified 2026-07-28

A customer account reaches the milestone that earns a physical mailing, a renewal notice going out by letter

Verify Address validates and standardizes the mailing address and returns a deliverability status

The agent routes undeliverable addresses to a correction queue instead of printing to them

Create Contact saves the verified address as a reusable contact for future mailings

The mailing owner reviews the rendered letter proof and the recipient before anything prints

Send Letter creates and sends the physical letter from the approved template

The letter ID, recipient, and status log to the mailing tracker and the team channel

What This Integration Enables

PostGrid makes physical mail programmable, and that changes what mail is for. When a letter requires a vendor cycle, you only send campaigns; when it is an API call, mail becomes something a workflow does the moment an event earns it: the compliance notice when an account changes, the welcome letter when a customer onboards, the postcard when an order ships. FlowRunner agents run that whole discipline, verification first, address book maintained, test mode in development, and treat the moment ink meets paper with the respect an unrecallable medium deserves. - Mail welcome letters, invoices, and notices triggered by real workflow events - Send personalized postcards to a segment, each rendered from HTML per recipient - Verify and standardize every address before storing it or mailing to it - Maintain a reusable address book of verified contacts across mailings - Develop safely in test mode, where mail is generated but never physically sent Test mode deserves its own mention as an orchestration feature. Because live and test behavior are separated purely by the API key, an entire mailing workflow, verification, contact creation, rendering, sending, logging, can run end to end in development with zero paper produced. Teams rehearse the full flow until the tracker fills with test letters that look exactly right, then swap the key. The first live mailing is the hundredth run of the workflow, not the first. Physical mail cannot be unsent, unprinted, or corrected in flight, which makes it the natural home of FlowRunner's [human-in-the-loop](/concepts/human-in-the-loop) gate. The agent handles everything paper does not forgive; the person owns the moment it prints.

Without FlowRunner

Mail merges and prayer Someone exports a list, runs a merge, and finds out about bad addresses when envelopes come back

Every mailing rebuilds the list Addresses live in whatever CSV the last campaign used, in whatever format it used

Print vendors mean lead time Physical mail requires a vendor cycle, so nobody sends one letter, only campaigns

With FlowRunner

Addresses verified before ink Every recipient passes standardization and deliverability checks before printing

The address book is an asset Verified contacts persist with IDs and get reused across every future mailing

One letter is a workflow step A single triggered letter costs one action call, so mail becomes event-driven, not campaign-driven

Use Case Scenarios

The thank-you postcard that ships itself

A [Shopify](/integrations/shopify) order crosses the loyalty threshold. The agent runs Verify Address on the shipping address, renders the postcard front HTML with the customer's name and product, and calls Send Postcard. The customer gets something physical in a week that no inbox filter touched and no scroll buried. During development, the same flow ran on a test key, generating every postcard without mailing one.

The compliance notice with a paper trail

A policy change requires written notice to affected customers. The agent pulls the segment from [HubSpot](/integrations/hubspot), verifies every address, and stages the batch for approval. After the human sign-off lands, Send Letter fires per recipient from the approved template, and each returned letter ID logs to [Google Sheets](/integrations/google-sheets) beside the recipient and timestamp. When someone later asks whether notice was sent, the answer is a row with a letter ID, not a memory of a mail merge.

The address book that cleans itself

Every new customer address entering the CRM flows through Verify Address. Standardized, deliverable addresses get saved with Create Contact so future mailings reference a contact ID instead of re-keyed fields. Addresses that fail verification go back to the account owner in [Slack](/integrations/slack) with the correction details PostGrid returned, while the fix is a conversation instead of a returned envelope. By the time anyone wants to mail anything, the list is already clean, and the campaign planning starts at the send decision instead of the data cleanup.

Human-in-Loop Highlight

Send Letter puts paper in the physical mail stream, and nothing in software can bring it back. A wrong-template batch is not an incident channel apology; it is hundreds of printed letters with your letterhead arriving in mailboxes for a week, each one paid for. Worse, letters carry weight email does not: a mistaken past-due notice or an erroneous policy letter reads as official. So FlowRunner treats live-key sends as a two-step act. The agent does everything up to the edge, verifies the addresses, renders the content, produces a proof from the exact template and data that will print, and presents the batch: this template, this many verified recipients, this cost. A person looks at the proof and approves. Test mode exists so agents can rehearse the whole flow endlessly; the live send happens once, on a human's explicit yes.

Agent processes routinely

Detects exception requiring judgment

Clear match Continues automatically

Ambiguous Routes to human via email

Human decides

Agent resumes with decision

Agent Capabilities

6 actions

Address Verification

Verify Address Validates and standardizes a mailing address, returning the corrected USPS or Canada Post form, a deliverability status, and correction details. The mandatory first step before any address is stored or mailed to.

Contacts

Create Contact Creates a reusable saved address referenced by ID as sender or recipient in future mailings, ending address re-entry. Returns the generated contact ID.
List Contacts Retrieves saved contacts most recent first with skip and limit paging. Used to find a contact ID to reuse.

Letters

Send Letter Creates and sends a physical letter from a template ID or raw HTML, with color, double-sided, and Express options. Test keys generate without mailing. The unrecallable send this page's human gate exists for.
List Letters Retrieves letters most recent first with status, recipient, sender, and creation details. The read behind mailing trackers and delivery follow-up.

Postcards

Send Postcard Creates and sends a physical postcard from front and optional back HTML in common sizes. Test mode generates without mailing.

Frequently Asked Questions

What can FlowRunner do with PostGrid?

FlowRunner agents can run Send Letter, List Letters, and Send Postcard in PostGrid, plus 3 more actions.

Does connecting PostGrid to FlowRunner require OAuth?

No. PostGrid connects to FlowRunner with an API key, no OAuth flow required.

Can PostGrid trigger a FlowRunner workflow automatically?

PostGrid doesn't currently expose triggers in FlowRunner. It connects as an action step inside workflows started by another trigger.

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