Address Validation works by using UPS or USPS servers to validate address, It means if the user enters address, our extension will use UPS/USPS servers to validate such address, and no warranty that this address will be correct for FedEx system and vice versa, if you know that some address is valid for FedEx, it may not be found on UPS/USPS server and our extension will return such address as invalid. We do not have FedEx validation. In this case you should allow extension to allow checkout even if address is invalid (it can be configured on extension settings page)
Our extension validates addresses right after user enters it. If you use magento standard checkout so address will be validated when user will enter address and click on 'continue' button.
Our extension is developed to work with magento standard checkout, our IWD One Page Checkout and the latest version of AheadWorks One Step Checkout.