One other issue with labels is where you place the label on the box.
If the label is placed on the shipping box on the seam where the box flaps meet, there is a potential that wrinkle or creases with be made in the label from the movement and box stresses while being shipped. To avoid this, make sure the label is placed so that the key areas of the label (address and barcode) are not along a box seam.
This potential problem may happen with taped labels and to a lesser extent with labelopes.
I prefer Canada Post free labelopes whenever possible over taping.
I also use a laser printer rather than an inkjet printer. I use a laser printer for years over an inkjet printer even before I sold on eBay. A laser printer minimizes issues with the label being damaged when wet even if the label is placed in a labelope or taped on. Water can seep in where you least expect it to.
As for personal mail received, I live in the Vancouver BC area where it rains a lot in the winter months. I have had wet (drops of rain) mail very rarely (1-2-3 times over the years) because the mail persons are careful with keeping it dry with their rain gear and mailbags. Same for the occasional parcel I receive.