The scheme, which Waitrose claims is a "game changing" initiative, enables customers to save 20% on 10 products they wish to save on each month.
The move helped contribute to a sales increase in the 12 weeks to 16 August, according to Kantar data, and the retailer is adding an additional 250 products, including babycare, health, beauty and 'free from', as it looks to expand the offer.
At launch, managing director Mark Price said that the retailer was introducing "mass customisation in grocery" with the scheme.
"We know our customers like straightforward offers they can trust and that are relevant to them - and their incredibly positive response to Pick Your Own Offers demonstrates exactly that," said marketing director Rupert Thomas.
"The idea of saving regularly really appeals to shoppers, with many selecting items that they usually buy to generate on-going savings."