I wouldn't trust any form of electronic communication for anything like this. It's not nearly robust enough. I'd use post. Like, printed stuff on actual paper.

I wouldn't trust any form of electronic communication for anything like this. It's not nearly robust enough. I'd use post. Like, printed stuff on actual paper. If you send something to their business address as registered mail, not only do you get proof that you sent it, you also get proof that they received it. It's much less likely to be ignored.