Your invoices are matched to your POs based on various criteria, such as effective and expiration periods, AWS seller entity (“Bill from”), line item type (e.g. AWS subscription, AWS Marketplace, AWS Monthly usage, or ALL), and line item start and end periods. PO balance is not an important criteria used for determining invoice association. When your PO runs out of balance, and in case you have not taken any action, such as updating the balance or adding a different PO, your PO will continue being associated with the same types of invoices as before.