First off, nobody pays sticker price. There's close to $2,000 of wiggle room in an Altima. Then there are the incentives. There are 2 factory to dealer incentives of $500 each and another $500 if you finance through Nissan Motors Acceptance Corp, for a total of $1,500 there. So, figure $20,900 (for a good negotiator) less $1,500 in FTD incentives for an out the door price of $19,400.
On balance, they did give you a reasonable deal on the extended warranty, not that it's worth all of the hype. The factory sticker price on the warranty is about $1,800 and you got it for about half of that.
You're stuck with the deal that you made, but you can cancel the extended warranty for a cash refund of the purchase price for it shown on the invoice. The procedure for that is explained in the extended warranty book that came with the car. You get a full refund in the first few months and it's pro-rated after that.
So, yeah, they basically saw you coming.
BTW, the smart key is standard on all Altima models.