Did he pay insurance for the shipping? If not, unless you're willing to contribute to the repair out of the goodness of your heart, you have no legal obligation to pay a penny toward this repair. Once something hits the US Mail, UPS, or FedEx anything can happen. Because of that, it is outside your legal control and obligations. Plus, if he went from $100 to demanding $200, I think you're getting scammed cuz the guy does not want to pay for the repair AND the helmet. If he paid through through his credit card, then there may be some insurance coverage from there, but it's his responsibility to work that out, not yours.
He sent photos of the damage, did he send you photos of the box? If so, was there evidence of damage consistent with the damage to the helmet? Did he make a damage claim with the shipper? If he did, he should have a record of that, tell him to show you proof of a damage claim. If he can't produce it...well...life's tough. At that point, it's up to you if you're going to give him any repair money.
