I had figured that there must be some technical reason why you did the pay upfront and then true it up. Maybe there were some state legal business definitions about pre-paid cellular companies or something so that you were required to do it that complicated way. Otherwise, I just couldn't see any reason for the confusing way it was done unless you had to.
This is so much simpler to explain to people. I already did explain it the new way to people the first time because that's way easier to understand, and it works out almost the same. If they were actually considering joining, then I would explain that detail about the upfront side and would mention picking XS plans.