Is Costco willing to take it back for a return if it doesn't work?
Or will you have to deal directly with the "Importer" (IE Firman)?
Personally, if it was me, I would pull it out of the box the SECOND I got home and fire it up, load it for a couple of hrs..
DOAs happen and you won't know if it is DOA or working until you pull it out of the box and test fire it..
Warranty starts the second the sales transaction happens, July you are talking having a gen sit in a box never tested by you and if it doesn't work now it is your word against the store or importer when it comes to warranty coverage.
Be aware, Costco isn't going to repair it or offer parts, you will have to deal directly with Firman who is nothing more than an importer of Chinese made products, they also do not make the gens, they buy them from another Chinese manufacturer who makes them..
If you go ahead buying a Firman (or any other Chinese off brand) I would highly recommend you replace the cheap spark plug that comes with the gen with a better brand like Champion.. The ones that come with the gens are pretty cheaply made and often are a source of starting problems..
Put the oil in some fuel and let it rip for a while..
Then before shutting down, turn off the fuel to the carb and let engine run out of fuel.. The other option is to turn off the fuel, then drain the carb..
No fuel in carb and you can store the gen for a long, long time.
Leaving fuel sit in the carb can easily clog the emulsion tube in the carb (this tube has little tiny pin holes smaller than a sewing needle and is above the main jet). All Chinese Honda clone engines use the same Honda designed carbs and even Honda carbs are not immune to this tube clogging off..