The most useful thing to understand about buying a mattress on Amazon is that two different policies can apply to the same box. There is Amazon's own returns policy, which covers the order, and there is the manufacturer's sleep trial, which some brands honour independently of where you bought it.
AMAZON'S SIDE. Mattresses are covered by Amazon's general returns policy; Amazon does not publish a mattress-specific window or a mattress-specific fee on its returns help page. Start any Amazon-side return from Your Orders.
THE SELLER QUESTION. Amazon's policy notes that items sold by third-party sellers can carry different return terms than items sold by Amazon. Before you order, check the "Ships from" and "Sold by" lines on the listing — that single detail decides who you will be dealing with if the mattress is wrong.
THE BRAND TRIAL. Several brands that sell on Amazon also run their own trial of 100 nights or more. That trial is administered by the brand, not by Amazon, and usually requires you to contact the brand with your Amazon order number. If Amazon's window has closed, this is your remaining route.
WHAT TO DO BEFORE YOU BUY. Screenshot the listing's return terms, note the seller name, and check the brand's own site for a trial that covers Amazon purchases. If neither is clear, buy from the brand direct instead — a documented 100-night refund beats an ambiguous marketplace window.
CONDITION STILL RULES. Whichever route you use, an unstained, undamaged mattress is the one that gets collected and refunded. The protector is the cheapest part of the purchase.