Synthetics Trafficking and Overdose Prevention Act of 2017 or the STOP Act of 2017 This bill amends the Tariff Act of 1930 to make the Postmaster General the consignee (i.e., the entity financially responsible for the receipt of a shipment) for merchandise, excluding documents, imported through the mail into the United States. The Postmaster General must designate licensed customs brokers to file required documents or information for such shipments. The bill amends the Consolidated Omnibus Budget Reconciliation Act of 1985 to impose a customs user fee on postal shipments or any other item valued at $2,000 or less arriving at an international mail facility. The bill amends the Trade Act of 2002 to direct the Department of the Treasury to require the Postmaster General to provide for the advanced electronic transmission to the U.S. Customs and Border Protection of certain information for all postal shipments by the U.S. Postal Service (USPS), including postal shipments it receives from foreign postal operators. The Postmaster General: shall be liable for civil penalties for postal shipment violations committed by a foreign postal operator or the USPS; may be directly or indirectly responsible for discrepancies resulting from omissions made or false information provided by a foreign postal operator or the USPS; and shall ensure that all costs and penalties associated with complying with this bill are recouped from foreign shippers, foreign postal operators, or U.S. ultimate consignees.