Let’s say I write: “Just send me those documents as is”. Is that correct? Should the "as is" idiom be pluralised into “as are”, which sounds so wrong to me? Or are they both incorrect, in which case I may write “as they are”?
Merriam Webster's Dictionary of English Usage (see page 131) says that the phrase as is is always singular, regardless of the number of items. As are is incorrect, but there is nothing wrong with using as they are. I think as is is most commonly used to do with the sale of something. ("I bought a used car, as is.")