Dynamics GP 2013 has some improvements around Purchase Order Close
GP2013 Weekly Topics; Purchase Order Processing
Weekly Topic: Purchase Order Close – Inside Microsoft Dynamics GP
Sales, Purchasing & InventoryGeneral Ledger & FinancialsTips & Hidden FeaturesGP 2013
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.