After clicking on the link you will discover that seasons 3, 4, 5, and 6 are all 6 disc sets. Are these the later releases? Is the content the same? They are also on "call for availability" stock status.
That's kind of weird because the S3 and S4 re-releases have six discs but the S5 and S6 re-releases have five discs. I would guess that their listing is incorrect and that these are all the re-releases. Either way, $20 is a great price.
The re-releases dropped the bonus disc for S1, S2, S3, S4, S5, and S9. The re-releases of S6, S7, and S8 have their bonus material because the disc is split with the features and the last episode or two of the season.
EDIT: Even when the sets drop the bonus disc, the commentaries are still there.