And what do you mean by letter of guarantee?
It's a signed message from a public known staked address of your own, that gives the user proof that the address X was generated by your website and that the mixing session is real. This way, he has proof that he sent the coins to your address if you/any mixer tries to scam him.
A few mixers do that (e.g CryptoMixer, BestMixer, BitMix, etc...)
Here is an example I just generated from BestMIxer:
-----START SIGNING BITCOIN ADDRESS-----
1BestMixVhna91MkP7pKRtjej3bFq6Ze46
-----END SIGNING BITCOIN ADDRESS-----
-----START LETTER OF GUARANTEE-----
We hereby confirm that BESTMIXER.IO has generated a BITCOIN (BTC) address: 17rCyydfavKR6pHVLDNXA1LAzf8t9at5Jb to transfer the incoming amount (after deduction of the service fee) to the following address(es): 100% to 1NinjabXd5znM5zgTcmxDVzH4w3nbaY16L after 2hr. 15min. The service is valid for a transfer of 0.001 BTC as a minimum amount per single transaction and maximum amount of 997.073 BTC (received in the period from 2019 Mar 16, 00:01:40 UTC to 2019 Mar 17, 00:01:40 UTC). Our fee is 2.9218% + 0.00007345 BTC per each receiving address. Order:
https://bestmixer.io/en/t/2O1OnRwdce, BestMixer code: 1M1a3U6R67. Reserves for mixing: Beta Pool. This Letter of Guarantee is digitally signed by our main account: 1BestMixVhna91MkP7pKRtjej3bFq6Ze46.
-----END LETTER OF GUARANTEE-----
-----START DIGITAL SIGNATURE-----
G2v3cdhmdlLTNIT0CbSLu9zA2LOuoDMiVLkUsaKSaPFlB5nf2nW8o6oWHrG6HWnPXwjj9B1MDne+ufKMm5eiOww=
-----END DIGITAL SIGNATURE-----
The address "1BestMixVhna91MkP7pKRtjej3bFq6Ze46" which message was signed with is owned by them and it's publicly known.