The rarest SNES is.... Star Fox Super Weekend. According to IGN, "The altered start-up screen displays 'Official Competition Cartridge'. Depending on the points scored, players could win a t-shirt, a jacket, or even trips to international destinations. An estimated 2000 cartridges were made. I saw one on E-Bay going for over a THOUSAND Dollars! :O
When you say concealed cash, do you mean cash sent through the mail? I would, but I prefer PayPal, and sending the shipping fee payment that way will cause a significant delay in when it's mailed out. Still, that's up to you. A money order, I wouldn't accept.