Translation:Do not take the most expensive couch of all!
"Allra" is used to emphasize that there really is nothing that is more "something" than that particular thing. It may seem superfluous, but you could look at it as the "really" for superlatives.
You could say "it is really expensive", but if you think about what you say here the "really" just points out that you "really" mean what you're saying.
In Swedish, "Allra" has a similar function for superlatives. So "it is the most expensive one" would be translated as "det är den dyraste", but if you want to point out that it really is the most expensive one, you would add "allra"; "Det är den allra dyraste".
Does this make sense to you?