I'm a new user (just signed up within the last month), and I'm just now coming to the realization that the billing tiers don't work the way I originally understood. It may be a bit harsh to say but I think the lack of information about the billing actually turns it somewhat deceptive.
When I signed up all I heard was "pay for what you use". That is actually a half truth. What should be advertised is actually "pay for a specified tier of use" (whether you use all of it or barely any of it). All along, I had gotten the impression that if I used a fractional amount of a tier, I would be charged a prorated amount. There is not really any information available (apart from in forums, which I doubt you can expect that all users frequent regularly) to provide details about how billing actually works.
My thought about the billing just seemed obvious to me because the plans page that I read when I signed up said:
"Select your plans based on how many minutes, text messages and megabytes you think you will use. That's what we bill you. Next month you will be credited or charged the difference based on your actual usage. "
So my proposal is that perhaps at least some verbiage can be put on the "Plan" page. Make it clear to users that if you use 100 minutes, the cost is $3, but if you use 101 minutes, the cost is $9. There is no fractional charge; that 101st minute will cost you $6. You are not really charged based on actual usage, just what lump of usage into which you fall.