Tremendous shout out to sincerely one of the coolest Quora dudes and answers I ever saw. It didn’t solve my immediate problem but I think his unapologetic matter-of-fact attitude improved me in a profound way. He gets right down to brass tacks. He talks like a shop teacher
My answer would have been that you sneak the underwear into the changing room with some decoy clothes and try them on over your underwear. But I am blinkered like a horse and live a narrowly-circumscribed life with no sense of wonder