That whole shenanigan with the dealership and even locksmiths frustrated me so bad... The first time I had to have a key cut I got lucky, they cut it for me at Home Depot but they're really not supposed to cut customer supplied keys... After that and a couple more back-and-forths I spent a few hundred and bought my own key-cutting machine, now ... In order to cut a key I order my own keys so for reference the key in question is referred to as a "GM circle plus" key (or Chevrolet circle plus). That is the key we're dealing with here, I thought I'd mention it as it may help someone in terms of reference material.
Granted even with your own key-cutting machine you still need at least one original.
Ok, so the way I did that with my 2013 GMC Sierra is I literally held the two black circle+ parts together with the replacement key facing OUT as I inserted the original into the ignition, turned it on once or twice, started it, turned it off, then inserted the copy and voila, it really was that simple there... It was so simple I couldn't believe it, but this was also my 2nd go-round.
The 2007 Impala was another story, I lost the working key and all I had left was the "door only" key... So I had to do the 10+10+10 30-minute routine with that one, I will give you the link but you will want to put your car battery on a charger while this is going down and also make sure to cut off all accessories and your headlights!
You may think your battery is strong enough, I killed mine twice during this process and by the time I had everything recharged it took several days instead of 30 minutes but this is the best video watch the whole thing I followed it exact:
Me happy camper, sure the key cutting machine cost me too, all in all I spent as much as going to have them made but it was out of principle to me.