What you had was indeed what in law is considered a negotiable instrument, as checks almost always are. The test is not whether anyone can cash/use it. That is a test for whether or not it is a bearer instrument. But all of this is irrelevant. The IRS prohibition described in Pub 519 is against transporting what they define as "monetary instruments," and as your OP notes, the check you had is specifically excepted from that definition. You did nothing wrong. Having said that, cashier's checks are treated differently than the usual personal check for certain purposes, and it is not a great idea to use them when not needed. International wire transfers don't need to cost "hundreds of dollars." Maybe $25 instead of $10. Currency conversion is a separate issue, independent of whether the instrument is a check or a wire transfer. The next time you need a large amount of money to be deposited to a foreign bank, have a wire transfer done. If your local bank says they can't/won't/don't know how to do it or that it will cost you a fortune for the wiring, find another bank.