People conflate three separate things: the store's return policy, the manufacturer's written warranty, and the warranties the law implies. Only the first is optional.
Return policies are the store's choice
No federal law requires retailers to accept returns on a product that works as intended. The posted policy is the deal, and it is enforceable as posted. Some states require policies to be conspicuously displayed and impose a default refund window when they are not.
Online sales differ in one respect: the FTC's mail order rule requires sellers to ship within the promised time or the time stated, or to offer you the choice of a delay or a refund.
Implied warranties apply automatically
In nearly every state, goods sold by a merchant carry an implied warranty of merchantability: the product will work for its ordinary purpose. A toaster that does not toast breaches it regardless of any return window. Where a seller knows you need something for a specific purpose and you rely on their advice, an implied warranty of fitness for that purpose can attach too.
These can be disclaimed with clear "as is" language in states that allow it, which is why that phrase appears on used goods. Where it does not appear, the warranties exist whether or not the seller mentions them.
Written warranties and the federal rules on them
The Magnuson-Moss Warranty Act does not require warranties, but governs them when offered. A warranty must be available to read before purchase, must be labeled full or limited, and must state plainly what it covers. Under a full warranty, repairs are free, within a reasonable time, and after reasonable attempts fail you can choose a refund or replacement.
Two provisions are worth remembering. A company generally cannot void your warranty because you used third-party parts or an independent repair shop, unless it proves those caused the failure. And "warranty void if removed" stickers are not lawful as blanket conditions on consumer products, a point the FTC has warned manufacturers about directly.
Credit card chargebacks
When a merchant will not resolve a genuine problem, the card network route often works faster than any of the above. Dispute the charge with your issuer, in writing, generally within 60 days of the statement. The Fair Credit Billing Act covers billing errors and undelivered or misrepresented goods, and issuers side with cardholders reasonably often when documentation is clear. Debit card protections are thinner, which is a quiet argument for putting significant purchases on credit.
Extended warranties, briefly
Extended warranties and service contracts are insurance products sold at high margin, which is why the pitch arrives at the register. Before buying one, check three things: what the manufacturer's warranty already covers and for how long, whether your credit card extends warranties automatically (many do, at no cost), and what the contract excludes. The exclusions are the product. Contracts that exclude accidental damage, wear parts and "commercial use" on a consumer item cover a narrow set of failures that were unlikely anyway.
The order to work through
Ask the seller, in writing, once. Escalate to the manufacturer under the warranty. Dispute the charge if neither resolves it. Complain to the FTC and your state attorney general, which builds the record that drives enforcement. Small claims court remains available for amounts that justify it.