My two cents: US is running a high volume business. If I'm them, I am discounting complaints made anonymously unless it involves some sensitivity around the identity of the person making the complaint for some seriously good reason (discrimination, personal crime committed against, etc). It gets to a point where if you're complaining but not willing to put your name to it without a very good reason for not doing so, it's not worth looking into. The reason being, anyone can make an anonymous complaint, and you wonder what the motive is for keeping anonymous.

I'm not saying don't do it, but if personal convenience is the only reason for staying anonymous, you might want to consider whether to put your name on it. Otherwise, I'm not sure that you'll get what you want out of it, as it sounds like you're envisioning people getting fired over it. If it's that serious, your letter needs to trigger an investigation.

From another angle: If it's that serious, why aren't you willing to take your business elsewhere? It sounds like you will continue on as a customer. From US's perspective, do they care?

Keep in mind, I am not judging you or your complaint. I'm just trying to hypothesize the perspective of the recipient.