List document id of documents status is done and document type is Paper and the document is shipped by shipping agent named USPS.
select document_id from documents where document_status_code = "done" and document_type_code = "Paper" intersect select document_id from documents join ref_shipping_agents on documents.shipping_agent_code = ref_shipping_agents.shipping_agent_code where ref_shipping_agents.shipping_agent_name = "USPS";