I like the gag under a mask idea though, we should use that somehow.
Maybe every third time you go shopping, be gagged under your mask? Ideally with a ball or ring gag, if you don't have a mask or long enough hair that hides the straps you may use a pair of panties that you've edged in before leaving.
You'll probably want to avoid people you know, they might wonder why you're not talking to them
When the weather gets colder, you should also add a Collar (ideally a posture collar) to your shopping outfit, you can conceal that with a scarf
By the way, did you have your first window orgasm yet? ^^