Yes,you will have the buyer's address.
But changing the advertised amount for shipping is tricky.
If your fee is too low, you have to stick with it. You cannot raise it.
If your fee is too high, you can use the Send Money service of Paypal to send your buyer a small refund. As I understand it, if you use the simpler more straightforward Refund link on the original invoice you will get a Defect on your seller account. I am willing to be corrected on this.
However, to reduce your labour and increase your accuracy you need a tape measure and a digital scale. Try Canadian Tire. I use a Starfrit kitchen scale since my items rarely are over 5 kilos.
Which brings up the other point. Use METRIC measurements. Canada Post went metric in 1974 and any others will be an approximation. Since a millimeter difference can move you from one rate to another, this is very important.
The Options link on the Sell Your Item form at the Shipping section allows you to change systems.
Use LetterPost as much as possible, along with its sister service Light Packet. These carry goods up to 2cm thick and 500 grams in weight. They are not trackable or insurable but are fast cheap and reliable. Don't use these for expensive goods.
There will be a rate change sometime next week for parcels. Ask the counter staff at your PO for the new rate card.
Or use Calculated Shipping. That will automatically change when the new rates come in.
