Don't order online with your credit card.
As Sainsbury's changed parking policy (£10 for 2 hours, free for 20 minutes), I tried online ordering.
I placed and paid the order with all items set to "no substitution". Sainsbury's substituted items, adjusted the price and automatically took a second payment from my credit card.
I filled in online refund request form but it said "The Order number you entered is not eligible for Groceries On Line order refunds."
I used the online chat and waited for around 30 minutes, then went to get a coffee and back in 5 minutes and saw their reply "It looks like you've stepped away from the chat, so I'll go ahead and close our conversation". I'm now waiting for their colleagues again (another 25+ minutes) and spending my waiting time to write this review.
It's horrible service level, much worse than Tesco / Morrison.