Rithum (formerly ChannelAdvisor) is a 3PL solutions provider that Poshmark has integrated with to allow for automated Order, Shipment Tracking, Product Creation / Edit Feeds, and Inventory Feeds.
Please note that in order to link to Poshmark via Rithum, you must already be an existing Rithum client. You can reach out directly to their team for further details on how to join their platform at https://www.rithum.com/
Additional details can be found in this overview video