The letter you sent is signed by 1BitmixUUEGGACPeKciTkxr6TaLUnYgekM, meanwhile our address is 1BitmixerEiyyp3eTLaCpgBbhYERs48qza. So it is not our Letter of Guarantee, seems user been scammed by one of phishing websites.
Oops! I must have messed up somewhere while I was verifying one of my own letters. The signed message is valid, but it is from that address and not the one by you. Anyhow, I'm glad that this has been cleared up. I've updated my previous post which stated that the signed message is valid (while in fact it isn't).
It is probably one of the websites that 'LoyceV' mentioned, although OP could possibly tells us the exact one that he/she used. Time to report.