Those options are greyed out and pre-selected based on my profile during the booking process.
Hmm, that's annoying. Probably the only way to do it then is the way you have been doing it--leaving it off and then opting in at the counter or exit gate at the lot.
You can also check Avis' mobile app as well as avis.ca. Sometimes one or both of those behaves a bit differently than Avis.com.