https://github.com/discourse/onebox/pull/457
Onebox does currently send a custom user agent when making requests on various Amazon sites. However, we’re sending a very old user agent which wouldn’t be commonly seen.
I’ve created a simple PR to update the user agent to a more modern one. This may help with the 503 errors.
Please note, however, that during manual testing of this change I couldn’t establish a clear pattern of what was triggering the 503s. In many cases, sending the old user agent would result in a 503 - but sometimes it wouldn’t. Therefore, I wouldn’t guarantee that this will fix the problem, but I think it will go some way to helping reduce the number of errors.