Error 503 Service Unavailable indicates an issue which arises when the server cannot handle a particular request, either because it is down for maintenance or due to an expected overload.

Fix Steam Error code 503 ‘Service Unavailable’ on Steam Error code 503 is an HTTP error, this basically means that issue is caused by the steam servers and not within your computer.

A look at a cURL of the site shows that Amazon's homepage is returning a 503 Error, which indicates that the server powering the site is temporarily down due to maintenance or overloading.