Configure your new carriers
If you need another carrier, get this configured soon in your shipping system. Test it as soon as possible.
If you use BigCommerce, this is as simple as logging into ShipperHQ (included with the Enterprise). If you use Adobe Commerce, you must configure FedEx or USPS credentials. This might be an ideal time to offload this processing to a 3rd-party system, like ShipperHQ.
The good news is if you use a system like ShipStation to fulfill orders, you won’t have to change much of your process. Simply select another carrier.
On another note, test the fulfillment process with your new carriers. Depending on your store's customization level, it’s conceivable that post-fulfillment functionality could be broken.