Having a similar issue as other posters - some (most) of my eCredits have no checkbox. I called in, they actually said they could convert it to something else which I'd be able to use online, but it'd expire 1 year from date of issue instead of end of 2021. Unfortunately, after all that hassle, I still can't use it to book online. At least I made a different booking while I was on the phone that time, so it wasn't a completely wasted call.