To avoid undercharging customers for FedEx residential surcharges, you can integrate FedEx address validation into Acumatica to automatically detect residential addresses and apply the correct fees during order entry. Alternatively, implement a custom rule that adds a residential charge to FedEx Ground shipments when applicable. Using the FedEx Rate Request API can also ensure live rates, including surcharges, are applied before order completion. Additionally, customizing shipping methods to prompt checks for residential addresses and clearly communicating potential surcharges to customers can help prevent discrepancies.