Final Billing Overcharges - where do i formally complain?

I recently (regretfully) had to port my cell service to another provider. This was necessary after taking the decision to move my broadband to a Fiber provider. I had waited almost 2 years hoping comcast would provide a similar service but that has not happened.
It was necessary to move cell service because Xfinity charges an exorbitant fee for mobile service if it's not paired with Broadband internet. So that's the why of cancelling all of my comast services after being a customer for 8 years.

The issue i now have is that Xfinity are gouging me on the way out. And having spent at least an hour on chat with an agent their hands are seemingly tied in their ability to amend the final bill.
Basically i am being charged $54 for 2 apple watch lines which were unusable after my cell numbers were ported. These lines shared the exact same number as the phones.
It's a combination of the $10 monthly line fees and then a staggering $17 per line for them being 'live' but unusable. For comparison the exact same pro rated fee for the 5 cell phone lines that used to be on the account was a mere $0.81c each. This is about the underlying principle. Whilst i can afford to take the $54 hit not everyone can, and i shouldn't have to. It's a stupid filing process that cannot adapt to what must be a pretty common situation. Lots of people have smart watches. When the paired phone line is ported to a new provider. It should be pretty evident that the customer is taking the watches with them - you can't have a watch line without a phone line. If one were cynical this could seem to be quite deliberate, and probably makes the xfinity business a tidy little sum each year as people shop around for the best deals.

It's a novel way to be reminded that customer loyalty is worth very little today. But strikes me as an unhealthy way to run a business. This has left a sour taste, and it amounts to burning bridges, because i doubt that i will ever consider comcast or xfinity for my business in the future (remembering that i was leaving as a customer regretfully. 1. because comcast are not keeping up with its competitors for internet service in my area, and 2. make holding onto mobile service financially non-viable by charging enormous monthly line fees for a solo service. That may well be a business decision. Perhaps mobile only accounts are not worth the operating cost of maintaining. However, its still not a great business practice to sour one's loyal customers on the way out, making it almost certain that they won't ever return. And all for such a paltry sum in the end.