All the people saying larger subs are free (24 for 12 etc). They used to be with most supermarkets but now many charge for the subs or if the offer subbed was on offer apply that reduction to the new item.
So something from Ocado on offer at £9 instead of £12 was subbed for a £20 item and £15 charged.
Or they charge you the full price of the substituted item if you accept it
From Ocado website:
All our special offers are subject to availability. However, if you order items that are on a multi-buy special offer and we're unable to deliver all of the Items you need to get the multi-buy discount, we'll give you a pro-rata discount instead. It'll be based on the price of the items we do deliver. Equally, if you order an item that's on special offer and we substitute it, we'll give you a pro-rata discount on the substitute item based on its price.
So, now for the trickier part - how do we work out the pro-rata discount? To do this we need three things:
The total cost, before discount, of the items on special offer that we delivered (including any substitutes).
The total cost, before discount, of the items on special offer that you ordered.
The saving that you would have received if we delivered everything you ordered.
Then we divide (1) by (2) and multiply it by (3) to arrive at your pro-rata saving. Finally, we divide this total between the items on special offer that we delivered, based on their cost.
Morrison
If you accept a substitute product that we offer in place of a product ordered, we will charge you the price applicable at the time we pick your order for you. The price will be set out in the email that we send to you to confirm that we have picked your order.
Sainsburys
If you have substituted items that are more expensive and you choose to keep them, you'll be sent a confirmation email. At the same time you'll see a voucher drop in your online voucher wallet. This voucher can be used on any future online grocery order within the next 6 months.
Tesco
We 'price match' our substitutions, so if we've substituted a more expensive product than the one you ordered, we'll only charge you the price of the item you ordered. Substituted products for link-save and multi-buy offers (such as Buy One Get One Free) and Meal Deal offers are not included.