From a quick Google search, I saw a guy managed to fit 7,400 dollar bills into a shoebox. $10,000 bills are no longer printed but still valid, legal currency. Assuming that's the max. That's $74,000,000. You could dupe the box and net $148 mil.
However, thinking bigger. Go with a shoebox full of perfect diamonds. The value could conceivably be in the billions.
Yeah, plus the diamond cartel probably doesn't need competition, they have a hard enough time keeping the rock relevant.
You'd be better off asking for shoeboxes of more common materials so you could skirt the inevitable questions on the matter and not get flagged as a counterfeiter or something. Though personally, the majority of things I'm interested in owning can fit in a shoebox anyways so why not cut out the middleman?