Is she an ugly, undesirable mutt?
Of course not! (There's no such thing.)
So other than the weather, you probably want to protect her from people. Sad, I know, but dogs are stolen every day.
Do a search for "lockable dog leash" and you'll find some options. I can't recommend a specific one as I've never used any... but I'm sure there's something that will work for you and not be too expensive.
If your dog is especially "valuable" -- a rare breed, particularly beautiful and/or popular, even a lockable dog leash may not be good. (A sharp knife can overcome most of them.) If you think your dog might be a tempting target, it's probably best NOT to leave her alone while you're in the store, even if it's just for a few minutes.
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Diogi
"A Trained Mutt is a Happy Mutt!"