Toll roads in Norway- surprise!

If there was a European-wide competition for the least intuitive method of toll collection, Norway would win the prize. So, if you plan on driving a car in Norway without doing much prior research (as I foolishly did today), don't let their toll system surprise you. To describe, I was driving towards Trondheim and I saw a sign that read something like "Automatic Toll collection, 5 km". I assumed this would be similar to the EZpass system used in the US, and that you would have the option of paying at a booth. Well, after fishing through my pocket for the correct amount of kroner, 5 km had passed and I didn't see a booth. Hmmm, that was strange. About 15 km later, I saw an additional sign announcing another automatic toll collection. Once again... I drove along and didn't see a booth. I took the same route back, and this time I noticed the sensors that spanned over the road. OK, so they're obviously noting each vehicle as it passes, but how the hell do I pay? A little after-the-fact online research revealed that you either have to pre-register your vehicle online and buy credits, or you can leave the motorway immediately after passing the toll point and pay at a petrol station. If you do neither, you will receive a bill in the mail at a later time... which, I assume, I will... I doubt Norway is actively trying to entrap unsuspecting foreign drivers, because being particularly isolated from the rest of Europe, foreign drivers probably constitute a tiny minority of those on the roads (unlike Germany, whose roads seem to be clogged with the motorists of every country in Europe!).