Please reach out to Shopify Support to enable "Third-party calculated shipping rates" prior to enabling real-time rates.
Setting up Real-Time Rates for an Existing Shopify Store
Log in to your account at https://app.shiptime.com
Click the button to manage your Shopify Store.
Under Checkout Rates Click
Input some default dimensions. You can change these at any point so just use your most common or make one up!
Once connected your Shopify Store should now have Real-Time Rates enabled!
Once the above is completed you will want to make sure you go into the Settings - Products screen in your ShipTime account and edit/add weights for each of your products. If you chose Packaged items you will also need to go into the Settings - Boxes screen and add custom boxes.
If you have issues, errors, or questions, please email: firstname.lastname@example.org