You need a scale to weigh your parcel and a tape measure to learn the dimensions. These should be metric. Canada Post went metric in 1974. Any other system will give you approximations.
Because eBay is an American company, you will find the metric option in the dropdown under the Options link in the Shipping section of your Sell Your Item form.
Go to your PO and get the pamphlet on letter and package rates. Make sure it gives the maximum and minimum dimensions for each service.
Ask the clerk for Air Mail stickers. Ask her for Customs stickers. Not only will this be useful, but you will learn how helpful the local PO is likely to be. One of my local PO will only hand out Air Mail stickers one sheet at a time, and one customs sticker at a time. So I go to one that gives me as many as I want.
Finally, remember that the delivery person will be taking all your customers mail, folding it inside the largest piece and holding it together with a rubber band. If your item should not be folded use either cardboard or coroplast as a stiffener to prevent it. Just writing Do Not Fold is not going to help.
