How can I find out how much it would cost to ship a 3 pound laptop by USPS?

I’ve gone to their website and I don’t get how the hell I’m supposed to use it to find out the answer, so could someone who’s done this kind of thing before help me?
If it’s relevant, the shipping box’s dimensions ought to be about 12 by 12 by 4 to make sure the item fits.
Can I just wrap the thing in paper with an adress and stamp on it instead of looking for the right kind of box? I’m trying to list this thing on eBay and they’re giving me like a thousand options for how to tell customers the thing will be shipped, most of which I know nothing about.
So, if there’s an appropriate flat rate package box, what would the cost of it be?
And if I can just wrap the thing, about how much would be the stamp cost?