The Dollar Wallet basic is made of fiberboard, it is very flexible and will not break under almost any circumstance.
The Dollar Wallet plus is made of aircraft plywood, which is the plywood used to make model planes. It is some of the toughest thin wood on the market, which is why we've selected it. It is firm enough to provide a solid backing for your wallet, but it is also extremely pliable and will not break except under extreme conditions.
My prototype bends by about 20° easily against the grain without any trouble and returns to its perfect straight shape. With the grain, it bends to about 40° easily, but stands the best chance of breaking in that direction, that's why we've made the long side of the wallet, the one with the most potential for bending, against the grain. It sounds counter-intuitive, but the extra rigidity combined with the pliability makes it much more likely to bend without breaking.