We've discussed the tax writeoff ability of these before. In for a major IRS headache if you think you are going to deduct these.
Without the paperwork yes, but HI should supply a receipt with total amount spent and an estimate of value (if requested) for the item received. I'm on a board of a local charity and we do this all the time, especially when someone donates art for an auction. How much would it cost to recreate the item specifically for HI? If there is no comp available, then it could be Adlard's commission rate + all other associated fees = fair value. The burden of fair value should be placed on HI to create, that way the buyer can claim the excess.
If they want bigger donations then HI should be offering this.
Specifically for the Adlard, I doubt anyone is getting a deduction on that. You would basically have to prove that it is worth more than the $12,100 it ended for. I don't believe his commission rates are that high, and maybe the OA collectors can chime in, but his pages don't go for that much, do they? Commissions and actual OA pages from the book are probably reasonable equivalents for determining valuation.