I have put the GM solenoids in my 2.2 about a year and a half ago and no complaints or P0013 and P0011 codes since.
Did one of the solenoids, three months later a code pointing to the other one popped up so i changed it then, aprox $35 US each + Postage both times.
A local Alfa Mechanic wanted to sell me one for $260 + 1hr labor, at that rate would have cost me $600 - $700 AU easily.
Are you sure your's were fitted right and nothing other was disconnected along the way? You'll come across the path of the camshaft sensor and coil pack when removing the red plastic engine cover.
It's not really a mechanics job given those solenoids are a 5 minute job to replace - are you sure he's not stuffing around to get some extra money (and trust) out of you or to sell you more expensive parts in the future? Just a suggestion, i'm not pointing the finger at anyone here but.