Well, Person sunglasses are the hallmark of quality and history when it comes to eyewear. The classic lines, upscale design, material composition and the protection from ultraviolet light make Persol sunglasses a top choice among eyewear connoisseurs. It seems that the sunglasses you bought are larger than your head size, therefore, it would easily slip off your eyes. My advice for you is to ask for a replacement to the company.
You just find a screwdriver to make the persol sunglasses be tight. You adjust the nose pad position by using the screwdriver. Then you could put it on and see whether it will slip off the eyes again. You could also bring the sunglasses to the eyeglasses store to ask the client to adjust it for you.