Integrating shipping APIs can be a tricky part of building or scaling an eCommerce platform. One of the most common challenges developers face is managing inconsistent rate calculations across different carriers—what's displayed at checkout doesn't always match what gets charged later. This can create frustration for both businesses and customers.

Another frequent issue is tracking updates. Some APIs push updates in real time, while others lag behind or require polling, which adds complexity to the logic and can affect the user experience. Address validation is also a pain point—many APIs have regional quirks, and handling edge cases like PO boxes or international formats can lead to failed deliveries if not properly managed.

Has anyone found a particularly smooth shipping API to work with? Or tips on dealing with these headaches more efficiently?