If you or your customer cannot check out on Shopify online store, there are two reasons that may casued the isuse.
Fristly, go to "Shopify-Settings-Shipping" and make sure you have created shipping zone and added rates as shown below.
Click "Manage rates".
Then, find the "Shipping to" tab, and click "Create shipping zone".
Choose the zone you need and click "Done"
At last, click "Add rate".
Secondly, please make sure you have added " Topdser" in "Shipping from" tab.
Pay attention to the error message on Shopify: No zones or rates.
Customers won't be able to complete checkout on orders fulfilled from this location.