I'm surely in the hundreds of rockauto orders over the last few years. Never once have they sent me the wrong parts. They send what you buy is my experience.
They probably do get it right 99.9% of the time.
I'm just curious in this case how much cheaper they are on 8 spark plugs. If a person is hundreds of miles from the nearest parts store it's probably feasible. Otherwise I doubt it.
At $5 a plug that's $40 per set. At even 20% off that's a savings of $8. If $8 is seriously that critical, I'd consider re-gapping the old plugs and milking them for a while. Just my 'over the top' opinion.
BTW...….Advance Auto Parts gives me a 20% coupon (if not 25%) every time I log on to their site.