TBH if the front rotor diameter is the only thing not quite as per the website spec' then you've probably done quite well for yourself.
Have you checked the model number on all the components?
I mean all of them? Are the BB, tyres, Front Mech, Hubs, spoke count, stem, bars, headset and seat post all exactly as per the spec sheet?
Then you probably need to ask yourself is a slightly smaller rotor than the website says an actual deal breaker or are you being picky for other reasons?
Is this really a bit of "buyer's remorse" with some transference onto the shop staff?
If you do take it up as a complaint what sort of outcome are you really looking for?
-A slightly bigger rotor to be fitted FOC?
-Some money knocked off?
-An apology from someone?
-Return of the bike and a full refund citing SOGA?
Those are the basic options, and chasing till you get any one of them is a bit Dickish TBH... and I'd not be shocked if future enquiries from oyu were met with a polite "We can't get those I'm afraid" when you get yourself labeled as a "Difficult customer".
Strictly speaking I don't think the shop are at fault here.
They had a contract with you to supply a "Giant Talon 0" and sell it to you at an agreed price, they've done that, the detail of the spec was probably not stated. If there were any items stipulated in that contract such as "the bicycle shall have a 180mm diameter front brake rotor" or even if you'd printed off the spec sheet and insisted they staple it to the order and write "Bicycle to be exactly as per this specification" then you might have a point, but that isn't what happened is it.
They got the bike delivered in a box from the distributor assembled it, checked it was functional and safe and then no doubt gave you a call to tell you it was ready, did they charge any extra for that?
For their trouble they get accused of dicking you about and then you head off to whine on the interwebz, and we wonder why LBS's are in decline...
I'd ignore a blatant chancer quite clearly trying to get something for nothing after the deal's been done.