When I want to make 「それを見せて下さい。」 more formal, I always say 「それを見せて頂けませんか？」. But, I want to double-check this.
I never say 「それを見せて下さいませんか。」. While that seems perfectly fine, I don't think I've heard it used very much by native speakers. Rather, I think they more frequently say 「それを見せて頂けませんか？」.
(-) Is 「それを見せて頂けませんか？」 really used more frequently than 「それを見せて下さいませんか。」?
(-) Ignoring any sort of "nuance", are there any cases where I cannot say 「それを見せて頂けませんか？」 and I must say 「それを見せて下さいませんか。」?