Itzel Mastery Vendors (Checked both Shipwreck Peak Waypoint in Verdant Brink and Teku Nuhock Waypoint in Tangled Depths) do not have a 5 Obsidian Shard option- both the [25 Airship Parts + 2,450 Karma] option and the [125 Airship Parts + 12,250 Karma] option are for a single Obsidian Shard.