You are a seller?
If you are using Calculated Shipping and metric measurements the price your buyer sees is the price they pay.
You need a tape measure and a digital scale. Canadian Tire has good scales by Starfrit for $20 or less.
If you are using Flat Rate shipping, go to the Canada Post site and read about shipping rates. There was a price increase in January and another on different services a few days ago.
Your buyer only has to pay the amount you advertise. If you set up shipping to say the buyer's cost is $4.83 and it turns out to be $5.29, you take the difference out of your profit margin. Do NOT ask your buyer for more. It is your responsibility to get it right.
And begging for more is a Violation of your seller agreement.
If your buyer overpays, for example if you advertise that shipping will be $5.29 and it turns out to be only $4.83, you can keep the money (to make up for all the mistakes that went the other way) or you can use Paypal's Send Money to make a refund. Remember when you do this that eBay will charge your Shipping Final Value Fee on the amount you advertise and will not give you credit for any refunds.
There are a few instances where the seller sets up shipping, but the buyer cannot see it. The most likely is when you are using Calculated Shipping and the minimum USPS dimensions are a little larger than Canada Post's dimensions for similar service. If you add a centimetre to one dimension this will correct itself and the rate will not change, but will be visible.
