The traffic isn't completely circumventing your last mile ISP. Just because you tunnel over that connection doesn't mean they're not serving your traffic.

Likely the outcome would be "we couldn't identify this traffic as one of our approved providers, so we used the lower of the rates we offer."

e: the second bit of that was to compare to the plan offered by a peer comment, with e.g. netflix, youtube served at higher rates and the rest at lower. So you would never see the higher rates.