Verizon and Debt Collection

I need a little advice. In February I cancelled my Verizon home account and switch to another provider. I had been using the Verizon One Bill system to pay both my cell and home bills. On Feb 22nd, they billed me for $173. On March 1st, I logged into the online account, saw the amount due of $173 and paid the bill. No more was ever said or received from them. I thought it was done.

In July, I received a call from EOS CCA saying that my account had been turned over to collections for $150! Needless to say, I was a little concerned. I called Verizon since my account had been closed for months and I had no access to their online bills. They said that on Feb 29th, they billed me for $150 that I never paid. I have a printed, dated screen capture of the March 1st bill showing only $173 that I payed, how can they hold be liable for a bill that I A.) Never received and B.) was not listed on the March 1st online bill?

So, I wrote EOS CCA a letter with all of my findings and enclosures and sent it certified mail asking them to forgo the wrongly incurred debt and to remove any mention of this from my credit report.

I never received any further calls or correspondence from them, so rather ignorantly, I assumed the matter was closed.

Now to the present. As my New Year's Resolution, I decided to change my financial status and work on my other debt. I got all of my credit reports as part of my semi-annual review and was shocked to see that my score had drastically dropped and now there was a derogatory statement with a major past due amount and a Collections tag on my Verizon account.

I've submitted dispute claims to all 3 bureaus and so far Experian has said that the debt is true and will not change it.

What can I do to eliminate the bad mark on my credit. I am not willing to sacrifice my credit and potentially my military career over a measly 150 bucks! I'd gladly pay it, even though I don't think I should have to, if that is what it takes. Any ideas of suggestions would be GREATLY appreciated.