Hello,
Thank you for contacting us about a 503 error using Shipstation in a PrestaShop store. We are happy to help you troubleshoot, but will require some additional information.
What version of PrestaShop, and the Shipstaion plugin are you running?
Since this is a commercial plugin, have you tried contacting the developers? They may be familiar with this error, or have a workaround.
Have you checked for additional errors in your cPanel? Enabling error reporting in PrestaShop may provide a more detailed error.
If you have any further questions, feel free to post them below.
Thank you,
John-Paul B