I think it depends on the supermarket rowyn. I've been online shopping with Tesco since it first started getting in for 20 years ago. If anything is substituted and is more expensive, you don't pay any extra & I think you get a refund if it's cheaper but it rarely is so I can't remember. I thought all supermarkets did the same but DDIL has recently used both Morrison's & Sainsburys & said she'd been shocked when the latter had sent some very expensive steak instead of the frying steak she'd ordered and she'd had to pay the not insignificant difference &, unlike Tesco, she wasn't allowed to refuse it - with Tesco, if you're not happy with the substitution, you just tell the driver & he takes it away. Actually, thinking about it, he might not, given current circumstances. I understand any extra has to also be paid for Morrison's dearer substitutions. My sister is a Waitrose shopper & there seem to be a lot of perks so I'm sure they wouldn't charge for dearer substitutions!