You probably figured as much already, but for the benefit of anyone else who finds this: this email is spam. Ignore it. It is not a bill; you are not obligated to pay them anything, and, in fact, you shouldn’t, because they’re dirty spammers and deserve nothing (besides vicious mockery).
Generally speaking, ignore anyone who claims that you need to pay them to have your site “listed” with search engines. They’re trying to scam you — search engines will discover sites on their own, and do not require any periodic service to keep sites in their index.