Oy vey! For you and especially your wife! As a frum woman, I can definitely and unfortunately relate to the uncomfortable situation your wife is in. With all the ‘benefits’ of working and living in a ‘frum’ environment, breaches of tznius are unfortunately all too common. Here’s my advice:
1) ASK A RAV!! Even if you/your family is not the ‘sheila asking’ type, this is something that could use the objective chochma of a Rav. Without saying names, or even the name of the supermarket, outline the details of the situation. I find that talmidei chachomim have certain insights and suggestions that are so helpful when implemented. If you don’t want to say its your wife, say its your sister, daughter or mother, or some other female family member.
2) In general, I don’t think a ‘note’, anonymous or otherwise, would be so effective because unfortunately this shopkeeper probably doesn’t think he’s doing anything wrong. Honestly, he might just think he’s being friendly and doesn’t realize the damage he is causing. If that is the case, I would say that your wife might actually be the best person to have this ‘discussion’ with him. Subtle body language and verbal cues are effective in this way, your wife can distance herself physically if a conversation turns inappropriate, not make eye contact etc. Sometimes it helps to make it seem like your own problem, or to lighten the mood- she can say, “my husband sends regards” or “should we really be discussing this”? If he’s not getting the ‘hint’ after that, a casual mention to a supervisor may do the trick. I don’t think it would get him automatically fired (which may not be a bad thing), but rather a reprimand could come from a source of authority (his supervisor), without directly involving your wife.
and a bracha to anyone struggling with this yetzer that they should be able to overcome it!