I saw that, too. It's what eBay used to call a 'Dutch Auction'. The $75 you bid is on ONE box of film. When you place your bid, you can choose now many boxes you want to buy at that price, but it would be $75 for each box.
If you made a mistake thinking it was for all 2100 sheets, you can retract your bid and give the reason as 'entered wrong bid amount'. You can also e-mail the seller and ask him\her to cancel your bid because you didn't know it was a Dutch Auction - plead ignorance. Nothing bad will happen to you for retracting your bid - no matter what the dire warnings say.