I'm just curious why the default response is still to reach for an Arduino--much more powerful SoC chips are cheap these days.
You are sending people's orders around the web. I'd consider that "personal".
None-the-less, use SSL, there is little reason not to use it these days. And as others have pointed out, it's the only good and easy way to guarantee what you send to one of these printers is what it actually received (no carrier tampering of your packets, etc).
Just use SSL.
(I don't actually know how much work would be involved, but goleksiak says they would really like to use it, so I assume it's not trivial.)