Cox charged me with partial month service fee, which does not make any sense.

I upgraded to Unlimited Data Plan($89.99 + $49.99) from 1TB plus 500GB data plan($89.99 + $29.99) on 17th of December, 2018 via Cox website.  When I checked my account on the 24th of December, 2018, the Unlimited data plan was not applied.  So I contacted on the same day via Cox chat and applied again.  It was confirmed that the new data plan was applied and I will be paying only $20/month more from the previous plan. (I have a transcript of the chat sent by Cox.)

I received an email of billing statement on 23rd of January, 2019. (the billing cycle is from the 21st of December until the 19th of January.)  Cox charged me $89.99 + $49.99 for the unlimited data plan and an additional $43.00 for Partial Month Service fee.  So I phoned Cox on 24th of January, 2019 to verify what the Partial Month Service fee was for.  Cox told me that the Partial Month Service fee occurred because the new plan started from 24th of December instead of 21st of December.  And this will not occur from the next bills.  Cox told me about billing cycles, price protection, promotion, credits and so on, which sounds very complicated and ambiguous.

Since I have upgraded from lower price plan to higher price plan, not the other way around, I asked, "Wouldn't it be $89.99 + $49.99 is the maximum amount Cox can charge me?"   Cox answered, "Yes, but from next month".  So I asked again, "Wouldn't it be logical that the partial month service fee can only occur when I change my plan from a higher price to lower price?"  Cox answered, "I have explained more than once about your billing statement."  and started same words again and again.

Cox is the only ISP around my house.  What can I do?  $43 of one-time rip off is not much, but I hate being tricked.