Here are the webhooks that fire after a mailing action is taken on a document (e.g. W-9, Form 1099-K, etc.) and the subsequent USPS status updates.

Webhook Events

Webhook EventCause
MAILING_CREATEDWhen a mailing is successfully created for USPS.
MAILING_PROCESSING_FOR_DELIVERYWhen USPS has process the mailing, preparing it for delivery and its status is set to PROCESSING_FOR_DELIVERY.
MAILING_IN_TRANSITWhen USPS is actively delivering the mailing and its status is set to IN_TRANSIT.
MAILING_DELIVEREDWhen USPS has successfully delivered the mailing and its status is set to DELIVERED.
MAILING_DELETEDWhen a mailing request is deleted.
MAILING_RETURNED_TO_SENDERWhen the USPS could not delivering the mailing, has initiated the mailing to be returned to its origin and its status is set to RETURNED_TO_SENDER.
MAILING_UPDATEDWhen any property (typically the status) of a mailing request has changed.

Read more on how to register and validate webhooks here.