Reproduce the problem without changes
Write down the OpenCart version, theme, checkout module, cart composition, shipping country and sign-in status. Check if the problem is repeated in the private window and on the phone. Do not empty all visitors' baskets or disable modules in the working store for the sake of experimentation: a test copy is required first.
Continue with: OpenCart payments: verify callbacks and order status.
Match the browser and the server
In the Network tab, find the request after which the processing has stopped. Save its time, response code and secure error snippet without personal data. At the same time, find an entry in the OpenCart and PHP logs. HTTP 200 alone does not indicate success: the response may contain a validation message or HTML instead of JSON.
Continue with: OpenCart module updates: protect your checkout.
Test one hypothesis
On the copy, isolate the last change: theme, payment module, delivery or modifier. If the shipping method only disappears for a specific address, check its conditions and geographical zones. After correction, repeat the initial scenario, the adjacent scenario and the negative test with the required field blank.
What to pass on to the developer?
Page address, reproduction steps, error time and screenshot of the error. Do not include passwords and full payment details in the usual description of the task.