Thank you for this vital information.
While on the topic of ribbis there is another financial product with a big ribbis concern. Its called peer to peer lending or p2p. These companies give very low interest rates. The problem with this is the lending company is only a go between you and an investor who is giving you the loan. The investor or investors can be jewish. The investors also have the problem since they dont know who is getting their money.
Another serious shaila is these gas stations which are owned by Yidden and they charge an extra 8-10 cents extra if you pay by credit card (as opposed to cash). My Rav once mentioned that if we use such gas stations we should pay with cash only, in order not to be nichshal the Jewish owner.
The letter does not leave contact info. Firstly, it should do so for others to verify the letter. Secondly, who shall we contact if we have shaila. Such as : what do I do if I already closed a loan with them?