[mothered]

Just because It's scanned, It doesn't mean It was "packed".

Here's an analogy. You purchase an Item at the supermarket, the cashier at the register scans It, places It aside (to put other suitable Items Into your bag), and then forgets to give you the scanned Item.
Human error Is Inevitable.

So let's say I purchased a GPU and wrong item received a book weighing pretty much the same (0.05-0.09 out) that was stocked and fulfilled by Amazon, this would be possible?

Or am I likely to run in to issues due to the price discrepancy being like £460 between the 2 items 😂

The cost Is Immaterial.

As far as you're concerned, they've sent an Incorrect Item, regardless of It's nature and value. As long as they stock It, that's all that matters.