When an order requires (or is determined by error) to require additional funding to ship, the process for viewing the order is obfuscated. Here are the steps required–
ShipStation > Account Settings > Shipping > Carriers > Click the 3 dots beside USPS > View Reports > Click Balances > Switch to transaction view > Copy the tracking number > Go back to ShipStation > Orders > Shipped > Paste tracking number
ShipStation sends an email regarding the affected order. It should also include the tracking number associated with the adjustment.