Credit card companies will probably be the worst hit under your scenario, but they deserve it and it would solve the mounting problems of bad debt and over extended consumers. :)
Seriously though, sense most credit cards are issued by the bearers bank, direct payment seems reasonable there also.
One interesting question I have not seen answered is exactly how all these mysterious letters were delivered? Are we assuming that they are USPS delivered? Were some of them sent via courier or other means? I have not heard that the CDC has quarantined any Post Offices as a result of these incidents. If they have the letter and checked the postmark it would seem that they would want to check that post office. I suspect these letters were not part of a commercial bulk mailing and might not have actually been US Mail.
That would be an interesting fact to add to this whole situation.