When they ask what they can help with, just say you’re shopping for someone else and you’re taking pictures because you have no idea what they want. Pretty much act like a lost dog. Bonus if you’re a guy taking pictures of makeup that’s behind a cashier, “It’s for my girlfriend *rolls eyes* which one is mascara?”
I’ve read stories about people getting kicked out and detained by the police while using mystery shopping apps. Has never happened to me. This excuse is pretty plausible. While filling out info they’ll just think you’re texting the person you’re shopping for.
It’s also a really good excuse to tell the clerk to get out of the way while you take pictures of the stuff behind him.