Mini Box Self-Installation Charge?

Can someone please help me to understand my latest Cox nightmare???  I received my first mini-box in July and installed it with no problems.  I decided to get a second mini-box in August.  I also installed that box with no problems.  However, after a week or so, I had problems with the audio on the second mini-box as well as the lost of screen settings every time I turned on the TV (I would set the TV to 1080i but it kept reverting back to 480i each time I turned on the TV).  Being as this was a spare bedroom TV and because of my schedule, I was not able to look into the sound and format loss issue until September.  I phoned tech support and we went through a series of diagnostics both with resetting the box and with my TV.  We were unable to resolve the issue so a technician was dispatched.  He found the HDMI cable I used (which came with the mini-box) was faulty.  He replaced the HDMI cable and everything was fine.  I just received my bill and found a $39.99 service charge.  Being that the problem was due to the HDMI cable that Cox provided was the issue, I called Cox to have this charge removed.  I was told by the phone rep that she could remove only $24.99 but she couldn't remove $15 because I was never charged the $15 self-installation fee.   I had installed my equipment in July and August respectively and was never charged the self installation fee - nor had I ever heard of it!  If I had never phoned because of Cox's failed equipment - I would have not been charged!  Then I was told by the agent the Self-installation fee is now $20 as of a couple of weeks ago and I should be happy I don't have to pay the $20!  If the self-installation charge has always been in place, why did it take until my service call 2+ months later to have Cox realize their billing error???