Some key things facts I discovered digging through the website and phoning a Canadian consulate:
- you can apply in advance, even before you make a booking or plan a trip
- the eTA is good for five years, but ...
- if you need to renew your passport before then, you need to buy ($7 now) a new eTA because the eTA is linked to your passport number
- the eTA is stored in the Canadian government's vast databases so that you do not need any piece of paper to prove you bought one (of course, it's gonna be your problem if you say you bought one and may even have the printout but for some reason the database says no, you don't)
- it supposedly takes "minutes" to apply online and receive an email confirmation i.e. make sure you know where your airport internet access is or your phone has access just in case