A very safe public humiliation is making the slave ask for specific items at an adult bookstore. Make her introduce herself and say that her master sent her, and then have her ask specific questions about butt plugs, etc. For fun you can make her ask (while keeping a straight face) about their return policies on sex toys.

Much less of a threat of involving people who'd rather not be exposed to your perversions, since the clerks at most porno shops have seen it all.