Your contractor's invoice has charges you never agreed to

You owe what the contract says plus the changes you agreed to. Ask for each extra charge to be tied to one of those, in writing. Charges that can't be tied to anything are open to question until the contractor shows where they came from.

A homeowner hired a friend for a $12,000 bathroom. The work was poor, and then a new invoice arrived asking for more. Another found coffee, a ladder and a vacuum on their general contractor's monthly invoice. A third was billed for materials at a 20% markup and couldn't get the receipts. And one homeowner had the opposite surprise: the contractor marked a $16,000 invoice as paid, and 90 days later the money still hadn't left their account.

Check the bill against the contract

Fixed-price contract: the price is the price. Extras need a change you agreed to. "We found more work" without your yes beforehand is the contractor's risk, unless your contract says otherwise.
Cost-plus or time-and-materials: you pay costs plus a fee, so what counts as a cost matters. Tools and overhead are usually in the contractor's fee. Ask for receipts; a contractor billing costs plus a markup should be able to show the costs.
Allowances: if the extra is for selections over an allowance, ask for the arithmetic. See allowance overages.

Push back in writing

Pay what's clearly owed on time, so you stay on the right side of the contract. For the rest, write back line by line: "Item 4, $640 for subfloor: we didn't agree to this change. Can you send the change order or approval?" Keep it calm and specific. Many disputes end right there, because the contractor can't produce what they'd need.

Keep your own record of payments

The $16,000 story is a reminder to keep your own list: every payment, the date, the method, and what it was for, and to check that the contractor's record matches yours.

A message you can send

"Thanks for the invoice. We've paid the $4,200 for the second draw, as the contract says. We can't match three items to anything we agreed: $640 for subfloor, $180 for 'site supplies' and $95 for a ladder. Can you send the change orders or approvals for the subfloor, and tell us where the contract covers supplies and tools? We'll pay anything we agreed to as soon as we see it." It pays what's owed, questions only what's unclear, and puts the job of finding the paper on the person who should have it.

A total that only changes when you say so

When your contractor uses Everkept, the price changes only through changes you agreed to, each with its price shown before the work. Payments count once the side receiving them confirms them. You both see the same total, so the final bill is what you've been looking at all along.

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This is general information, not legal advice. States differ on what a home improvement contract and its changes must contain. Check with your state’s contractor licensing board or an attorney.