One thing strikes me as being very weird. They offer free lyrics to "over 60,000 songs" on their own website. If they give them away for free, how can they possibly say you can't?
I agree they need to get more specific. If they don't tell you which songs by which "members," they can't say it's "willful copyright infringement." I would also make sure the return address is valid, and try to contact Charles Sanders personally. I think it sounds like a scare or a practical joke.
Would you like to tell us their mailing address? I don't have much mind left, but I'm very willing to share a piece of it with them.
I owe you a contribution anyway - on behalf of myself and on behalf of Graham Pirt who just sent me a gold mine of Keith Marsden/Cockersdale songs and asked only that I contribute.