As someone who makes dry sarcastic jokes pretty often, I’ve learned you have to really put some ridiculous stuff in there to signal it’s a joke. This also scales with audience size and delivery method.

With so many people, you’d actually have to make the price ridiculous or something like that. Because some people, once they read that the printing is five cents, are going to be upset enough to not read the rest of the email.

I wouldn’t actually do this prank, but if I like had to, it would be more like the “charge” was to sing a song and the email would actually say April Fools in it. Maybe less funny, but a lot more easily seen as a joke. Makes handling the calls to the admins much easier, too.