Well, the good news is you now have a new battery and alternator. The bad news, it sounds like neither one needed to be replaced.
Did your mechanic at least zero in on the circuit causing the problem? That's normally just a matter of pulling fuses, until the culprit is found. It may then be more difficult to find the exact cause (broken/stripped wire, ...), but at least it gets you into the right ballpark. It sounds like the light circuit is the primary suspect, but if the '08 uses the same old infamous splicer, that would be another possibility as well.
I hope you can find a shop that can do this work right (i.e. find the battery drain), without hanging any more unnecessary parts (skip the stealership that sold you the alternator). This is another classic case showing why some of us try to DIY whenever possible.