You got ripped off if you ever installed ANYTHING without getting paid - interview or not, you always get paid man.

And no, most private shops don't care about MECP. Every shop I applied at gave me a very simple alarm to install and if you can do it as fast and as clean as they want, you are hired. I think you get a $1 - $2 pay raise if you have MECP as best buy and something similar at CC.