As far as I'm aware, this offer is only available to Amazon Prime Members and only members that have never had a subscription before.
UPDATE 27TH SEPTEMBER
Currently at the time of writing, these crisps now cost £7.02 on Subscription (when I purchased them, they were actually £8.53).
Using the Code 20SNSPRIME at checkout, you can get an extra 20% off your first subscription so the price you pay is only £5.61.... That's just 8p per individual Pack!
Great selection of crisps too
As you can see, I paid £6.39 for the 70 bags of crisps (as I said above, I paid slightly more because the item was listed at a higher full price when I purchased).
Terms & Conditions
From Tuesday, August 8, 2017, save an additional 20% on your first subscription of selected products when you enter promo code 20SNSPRIME at checkout (the "Offer"). This offer ends at 23:59 GMT on Sunday, December 31, 2017.
How to claim:Select any one of the selected products on this page (each a “Qualifying Item”) to subscribe. [See below on this page for the set of selected items]Choose the Subscribe & Save option on the product page, select your delivery quantity and schedule, and add the items to your Shopping Basket by clicking the “Subscribe Now” button. When you’re done shopping, click the “Proceed to Checkout” button.At checkout, enter 20SNSPRIME in the “Gift Cards and Promotional Codes” box. The Offer discount will be automatically deducted from your first subscription order.The Offer only applies to Qualifying Items dispatched from and sold by Amazon at the website amazon.co.uk. The Offer does not apply to purchases made from Amazon’s Warehouse Deals or from third-party sellers on Amazon’s Marketplace platform.Each customer can claim up to a maximum of 1 product subscription, and it must be claimed in one single customer order. This offer is only available to customers with a trial or paid membership to Amazon Prime or Amazon Student, and with no previous subscriptions.