Recover failed Stripe payments with Postmark dunning emails

Build me an agent workflow that recovers failed Stripe subscription payments by sending a personalized dunning email through Postmark and alerting our revenue team in Slack.

Trigger: incoming Stripe webhook on the invoice.payment_failed event. Verify the Stripe signature and only proceed on that event type.

When the webhook fires, hand the invoice ID and customer ID to the agent and have it do the following:

1. Use Stripe Retrieve Invoice to load the failed invoice. Read the amount due (in the smallest currency unit, convert to dollars for display), currency, attempt count, hosted_invoice_url, the last_payment_error code, and the linked subscription ID.

2. Use Stripe Retrieve Customer to load the customer's name, email, and any metadata that helps personalize the message.

3. Use Stripe Retrieve Subscription on the invoice's subscription so the agent knows the plan/product name and the next renewal date.

4. Branch on the failure code. If last_payment_error.code is requires_action (3D Secure / authentication required), the customer did not actually decline, so skip the Postmark send and post a different Slack message that says "authentication required, customer needs to confirm with their bank." If it is card_declined or any other normal decline reason, continue to the dunning email.

5. Use Postmark Send Email with Template to send the recovery email. Use TemplateAlias = payment_failed (not TemplateId, so the workflow keeps working if the template is re-created). The From address must be a verified Postmark Sender Signature; let me pick which verified sender to use during setup. Pass a TemplateModel that includes: customer_first_name, product_name (from the subscription's plan/product), amount_due_formatted (e.g. "$49.00"), currency, hosted_invoice_url (Stripe's hosted payment page), attempt_count, and next_retry_date if Stripe provides one on the invoice. Keep the tone short, friendly, and urgent. After the send, check the response ErrorCode and fail loud if it is non-zero (Postmark returns HTTP 200 even on logical send errors).

6. Use Slack Bot Send a Message to post to the #revenue channel (let me pick the channel during setup). Include the customer name and email, the MRR at risk (use the subscription's plan amount as a proxy if invoice amount is one-off), the attempt count, whether the dunning email sent or was skipped, and a link to the Stripe customer page (https://dashboard.stripe.com/customers/{customer_id}). One short Slack message, formatted with Slack mrkdwn, not a giant block.

Other requirements:

- Use the Slack Bot integration (xoxb token), not the user-OAuth Slack integration, since this is an automated workspace alert.

- Convert Stripe amounts from cents to dollars before showing them in the email or Slack message.

- If the customer does not have an email on file, skip the Postmark send but still post the Slack alert so a human can chase them.

- Log every run with the invoice ID, customer ID, decision (sent / skipped-3DS / skipped-no-email), and Postmark MessageID if applicable, so I can audit recovery later.