What should I do if I receive an unsolicited letter from an attorney in the mail offering to handle my case for a certain fee BEFORE they have ever spoken to me?

You should be very suspicious.  After all, this person knows nothing about the facts or your case or your background.  How can they possibly quote you a fee when they have no idea what work is required to resolve your case? 
This is why the Virginia rules of ethics strongly discourage the blind quoting of fees by attorneys before they have had a chance to speak to them about the case.
If you call one of these lawyers and they increase their rate above what is stated in their letter, you should immediately report them to the Virginia Bar because this is unethical.
If the attorney sticks to the blindly quoted fee, you should question what you are actually going to get from that attorney.   Every case is different.   Yet a fee that never changes says that the attorney is going to handle your case like every other case he has.   
What a lawyer charges is, in part, a product of four things: (1) his level of skill and experience, (2) his reputation in the legal community, (3) the time and effort he plans on putting into your case, and (4) the satisfaction of past clients.  MOST IMPORTANTLY a fee is a reflection of the effort a lawyer is going to put into your case. 
There are certainly those lawyers that abuse their ability to set fees by charging an overly inflated rate.  A lawyer should charge a rate that is fair and reasonable and allows for the appropriate amount of work to be done on the case - according to what is required on that specific case. 
I speak to prospective clients about this issue every month.  I recently talked to a woman and while this all made sense to her, she decided to hire an attorney that blindly quoted a fee to her in a letter that was one of the lowest prices. 
A month later I received a call from this woman and she was very upset.  She said that the attorney she hired showed up to court at the last minute and never did any real work on the case in court.  To make matters worse, this lawyer asked the woman to plead guilty to reckless driving at a speed that was not very high.  When the woman refused, the lawyer raised his voice and told the woman that she would have to sign off on a release.  The woman did, continued her case, and fired this lawyer.  The problem the woman faced is that many other lawyers, including me, will not enter a case after another lawyer that has mishandled the case.
Contact Manikas Law LLC today for a free consultation on your Fairfax case.  Kyle G. Manikas provides a free consultation for anyone charged with DUI or Reckless Driving in Fairfax County.