If you don't see knives like that often.. $20 is a great deal. Many importers dont like dealing with the carapo knives because they beat each other up in shipping and holding. lots of losses, inflates price.. or makes them harder to get.
I would try to get some pics and post them.. check out the Gymnotus species ID thread here in this forum, has a ton of great ID pics..
Carapos can be aggressive, Aba's are worse.. They might eat snails, but love worms.