I also have a 2.4 T5, and I have just now put my registration number into the Opie website and been told the shorter plugs are correct, which they definitely are NOT. See attached photo for proof that Opie are incorrectly recommending the shorter plugs.
I suggest you refer them to my link below, point out that their website is incorrect and tell them that you want to swap their incorrect plugs for the longer ones. Personally I've never bought spark plugs from Opie, but I have bought many, many gallons of oil from them over the years and I've always found them to be very helpful.
This is how I found for certain that the short plugs are incorrect -
https://www.volvoforums.org.uk/showthread.php?t=273646