Have you tried contacting the Boxify support team?
You can contact the boxify support team and they will help you to fix this issue.
An alternative solution for this issue is to use an app like the Ship, Rate, and Track for FedEx.
The app lets you update the rates quite often to make the shipping costs more reliable.
The variations in the rates from the carrier services are recorded by the app and only then the real-time shipping rates are displayed at the checkout automatically by the app.
If you have your own FedEx carrier account, the app lets you get the negotiated discount rates based on the volume of shipping orders.