We use an incentive-compatible experimental online supermarket to test the role of commitment and badges in reducing the carbon footprint of grocery shopping. In the experiment, some participants had the opportunity to voluntarily commit to a low carbon footprint basket before their online grocery shopping; the commitment was forced upon other participants. We also study the impact of an online badge as a soft reward for the achievement of a low carbon footprint basket. Participants from the general population shopped over two weeks, with the experimental stimuli only in week 2; and received their shopping baskets and any unspent budget. Results indicate that requesting a commitment prior to entering the store leads to a reduction in carbon footprint of 9-12%. When the commitment is voluntary, reductions are driven by consumers who accept the commitment. Commitments also reduced the consumption of fats and, for forced commitments, that of salt by 18%. Badges did not significantly impact consumer behaviour. Commitment mechanisms, either forced or voluntary, appear effective in motivating an environmental goal and search for low-carbon options, particularly in those accepting the commitment.