@Paul Cordingley just be cautious with any genuine branded products made to order by some Chinese subcontractor. The branded manufacturer will obviously use good quality control/assurance to get products that are up to scratch from their Chinese subcontractors. But the Chinese subcontracts that make the stuff for the branded company will either (1) manufacture more than what is needed (but with less QA measures because they don't intend to ship these to the branded manufacturer) or (2) have QA failed products on hand that the branded manufacturer has rejected (The ones John West reject). In either case, the Chinese subcontractors will offload such products though unofficial channels which eventually end up on eBay.
So the real problem is what are you actually buying? A quality tested product? Something that has been tested less stringently than the branded accepts? Something that has failed banded manufacturers stringent QA? Something else? Or are you simply buying branded packaging (considering the amount of obvious ripoff products floating on the www?
The reality might be that if the product is really cheap, it's cheap for a reason. Either you get no product support, no real warranty, no comeback or consumer protection. Really, it's best to buy from an Australian retail (unless the price difference is so great it's worth the risk).
So got to
Repco.
Hover over "Set my vehicle" icon towards right, then click on "manual search form" near the bottom of the pulldowm menu.
Select make, model, year, series, engine.
Then click on parts & service, then fuel ignition & exhaust, then spark plugs
Click on one of the two iridium plugs available.
You should see:
Champion Iridium Spark Plug - 9405 SKU: A9328475 $29.99
NGK Iridium Spark Plug - IZTR5B11 SKU: A9326706 $27.99
Next, order and feel good you are buying from an Australian business with all the included consumer protections and at a reasonable price
These should be good for either the 2015 L77 or LS3 V8 but check yourself
If you're still set on ACDelco plugs for your 'doore, then google "holden dealer acdelco spark plugs" and you'll find some Holen dealers pop up with rather varied prices, I've seen really cheap
$129 for a set of iridium's from a Holden dealer (had to google who they were)
Or check out
https://acdelco.com.au/auto-parts-stockist/ and pop in your post code and the nearest Holden dealers will come up (but you then need to ring up)...