$300 is nothing. Wait till a few more secondaries roll in, and then consider how much it is going to cost you to actually attend interviews. This is an expensive process, for sure. There was a thread towards the end of last cycle where people posted how much they spent in total, IIRC it ranged from $2k-$12k or so. Average was somewhere around 4 or 5k I think.
To answer the OP's question though, I think most schools are very indiscriminate in regards to sending out secondaries. There are exceptions of course (NYCOM only sends secondaries to interviewees, for example), but for the most part don't get your hopes up just because you got these in the mail.