Hi, and welcome!
I always recommend starting with local, common eggs for a first incubation so that you can verify your incubator is stable for the entire 21 days and so that you get accustomed to the rhythm of tweaking your incubator.
If it's a Little Giant 9200, with a knob to adjust temperature, then Amy will be your champion. She has had great success in hatching with one of those but she will tell you they need a lot of babysitting. I had two of them and sold them because they ruined more eggs than they hatched, though I am a set it and leave it person.
Still air with a turner poses a small challenge. The turner motor generates a good deal of heat, so don't place any eggs near the motor. Keep them two spaces away if possible. Likewise, the other corners are cooler, so try not to fill them. With only a dozen eggs, you should be able to group them around the center of the incubator.
If a hen laid the eggs in a nest one at a time while gathering her clutch, they would also go through those temperature swings. As long as they don't remain above 88F for an extended period of time, incubation won't start. When they cool at night, the average temp will also drop. And the packaging is most likely bubble pack and foam peanuts, which will further stabilize temperatures. I had eggs shipped from Montana to Michigan in below freezing conditions and they arrived at 60F.
When the eggs arrive, unwrap them gently and place them into cartons flat end up and allow to adjust to room temp. At the same time, get your incubator up to temperature and ensure it is stable. Let the eggs rest in the cartons for at least 24 hours. Candle them to ensure that none are cracked, and that the air cells are firmly fastened at the fat end of the egg.
You'll get better advice from those who handle shipped eggs often. I mostly hatch my own.
Shipped eggs are always a risk. Even if your results are less than stellar, don't let that discourage you. Try and try again...even those of us with a great deal of incubation experience sometimes have a bad hatch.