Municipal Auditing Services, LLC Telling me that I am conducting business at a friend's Smog Shop Internet

I received a letter stating that I owed 5 years of back city taxes for over $770 because they claimed that I was operating an unlicensed business in the city of Covina.  They said that they found my name on the real estate website with my mailing address, so they claimed that I operated a business in Covina, CA.  I just use my friend's address as a mailing address.  I have no business services, business transactions, or any type of business paperwork or products at my friend's smog shop.  

I haven't even done any real estate transactions in 5 years, and maybe only a total of 4 or 5 transactions in 10 years, all in the Inland Empire.  I stopped doing real estate a while back and just focused on fixing computers.  I mailed my tax paperwork to them showing my address in Rowland Heights for all the computer work I have been doing for the past 5 years.  Then they change their story and say I still owe to Covina because I might have done work in Covina.  It went from reason real estate, to maybe you might have done work in Covina as a business.  I'm not buying it.

I don't know what's up with these guys trying to find some way to get money out of me for no reason at all.