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

The PO wants to know the outer dimensions of the box (which is going to be larger than the dimensions of the laptop, because presumably you are going to pack it in something so it won’t get damaged)

They want to know the origin zipcode and the destination zipcode. If you don’t have one yet (if the item is still for sale) then pick a zipcode on the other side of the country, and say “Postage not to exceed $xx.xx)

They also want to know weight. Heavy stuff costs more.

Finally, they want to know whether to deliver it overnight (which will cost a bundle), or 3–4 day delivery, or parcel post, which can be 10 days. Cost changes depending on speed.

Usually on Ebay I let my buyer decide how fast he wants the item delivered, and let him pay the extra for fast service.

There are boxes for flat rate that might fit, but you need to insure it for what it is worth on top of that if they allow you to with flat rate. You’ll want to put bubbles or styrofoam peanuts around it in the box. Why not call or go to your post office and ask them, they are usually very helpful. You can call the 800 number also.

Here is the info and sizes on the flat rate boxes. your local post office will have some sizes on hand.

I think insurance is around a $1 dor $100 something like that?

Go to usps.com and find the link that says “Calculate a Price”. Enter the specifics that you’ve provided here, and you will get a very accurate price quote. You can choose different rates of shipping (First Class, Priority Mail, Express, etc) and between Flat Rate and by the lb.

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Okey just read the rest of your post. With 12×12x4, Most likely you will want to go with the USPS Priority Mail Large Flat Rate Box that has similar dimensions (there are 2 or 3 different versions of the box so make sure you get the one with the right dimensions). You can add a tracking number (“Delivery Confirmation”) and insurance if you want.