Clearly a SCAM.
Who in their right mind would send their Bitcoins to some wallet address and get a "letter of guarantee" in return?
This is a common way that mixing services prove that they own the address to which you're about send some coins. Obviously it's not a guarantee that the service won't scam you, it still requires you to trust that the service will return your money. It does guarantee that the service is in control of the address, however.
I've never used [banned mixer], so I don't know if they are a scam or not, but your comments make it sound like you've never used them either. Have you used the service, and were you scammed? Or were you scammed by imposters, like Jawhead999 suggested?