https://silversea.shiptoshoretraveler.com/Rental-Deposit-Arctic-Boots for boots.
It's actually cheaper to buy boots than to rent them if you've got the luggage space, and you'll then get to keep them for future use. We made space by using 30" REI rolling duffels and stuffing the boots with socks and underwear.
The parkas are free, and if the one you order doesn't fit, the ship should have plenty of extras.
Many persons on this board don't want to be bothered with schlepping boots back and forth and are willing to pay the exorbitant rental charge, but what the heck, the cruise is already absurdly expensive, so they'll tell you why not just rent. To each their own...
If you have an evening flight the next day out of Santiago, there is no need to spend an extra day there.