Q: Why are you charging for the comparison disk if the test was done without commercial motivation?
A: Because disk preparation, artwork, materials, replication, packaging and fulfillment don't come free.
BitTorrent _IS_ free (except for the bandwidth of the initial feed posting) and requires no further effort on your part. There is NO disk preparation, artwork, materials, replication, packaging, or fulfillment to worry about.
Please consider allowing the dvd to be posted as a torrent. It will probably lead to more subscriptions to your journal.