How do I ship a few books from Canada to the US?

When you list items on eBay, they lead you through the process for figuring the cost to mail. You should pack each item in a bubble envelope and weigh it before you do this so you can enter the final shipping weight. I add a dollar for those bubble envelopes, too. Then while it is for sale, each potential buyer can see how much they will have to add for shipping. When the item sells, you can print the postage at home and apply to the package yourself and drop off at your post office. They try to make it easy for you. For most books, postage to another country is so high that no buyers outside will be interested. I don't know if Canada has Media Mail, like in the US, but it is only for domestic customers, never for export orders. To buyers in another country, even books have to go air mail, at great expense.

Amazon is not quite so easy. I have sold books overseas and had to buy postage at the post office.

If you are not far from the border, ship them via media mail at a US post office. You will save quite a lot.