It all depends on how much stuff you order from Amazon. I use it quite a bit as they almost always have a "free 2nd day delivery" deal.

Yes, you have to cancel it after you use it before the end of the trial period or it's one of those subscription deals.

Bezos didn't get to where he is by giving stuff away.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now