I took a micro free tier from Amazon a month and a half back. But towards the end of last month I received a bill of ~$13 for Amazon services usage.
The webpage quotes
750 hours of Amazon EC2 Linux Micro Instance usage (613 MB of memory and 32-bit and 64-bit platform support) – enough hours to run continuously each month*
Can someone help me figure out if this is a billing mistake or if it's an issue with my understanding of “free”?