How the max bid is calculated
Most fee calculators start with a bid and tell you the fees. This one starts at the other end. It takes your expected resale price, subtracts the profit you want, transport, repairs, title and tax, and storage, and then looks for the highest bid where the bid plus the auction's fees still fits in what's left.
Because buyer fees go up in steps as the bid rises, a slightly higher bid can push you into a more expensive tier. The calculator checks this for you and rounds the answer down to the nearest $25, so the number you see already includes the fees at that bid.
What fees Copart and IAA charge
Both auctions charge a buyer fee that depends on the winning bid, usually a flat amount per price range and a percentage on expensive cars. Bidding online adds an internet or virtual bid fee, which can differ between pre-bids and live bids. On top of that there are flat charges such as a gate or service fee, an environmental fee and a title handling fee, plus daily storage if the car isn't picked up within the free days.
The amounts here are conservative estimates. They vary by account type, payment method and location, so always confirm them on the auction's site. If you know your exact fees, enter them under Advanced.
Why stress test a deal
Repairs often cost more than the first estimate, and resale prices move. The stress test shows what your profit would be at your max bid if repairs run over or the car sells for less, and gives a safer bid for that scenario. If a small surprise wipes out the profit, the deal is thinner than it looks.