Upvote:1
eTA is not a visa. You can use it to enter Canada as many times as you like while it is valid (unless something substantially changes in your circumstances). Usually it expires after five years or when your passport expires.
Also you do not need an eTA if you are arriving in Canada by land. It is only required if you are flying to Canada. You also do not need an ESTA if you arrive in the US by land.
Neither the US nor Canada require you to have any kind of permit in order to leave.