I've always shopped in Tesco and when they've needed to sub they don't charge for the difference, eg if I order orange juice for £1 but all they have is fresh orange juice for £2 they give me the fresh one but only charge £1. I've had an Ocado sub and the item is 80p more but the receipt doesn't make it clear, so I pay for the higher price? (Just so I know going forward...not being in excessively worrying about 80p!)