Buy any of the snacks in this offer: http://www.waitrose.com/shop/OfferDetails?offerId=13509284
And you can add a roll of Oreos to your basket free! (Usually £1.08)
Some of the snacks (like the Doritos) are also "half price".
It's worked for me on the original and strawberry cheesecake flavours, I've not tried the rest.
Available for delivery or collection until 03/01/2017