While shoppers increasingly value sustainable products, considering sustainability can be difficult and time-consuming while shopping. In an expert workshop with 22 stakeholders, we gathered requirements for an assistance system supporting customers in identifying the sustainability of products at the point of sale. We integrated the resulting demands in a first mockup prototype, which was tested and discussed with a focus group. From the workshop and the focus group discussion, we deduced a set of ten guidelines for sustainability-oriented assistance systems. These guidelines were transferred into a prototypical mobile application, which allows customers to specify their personal understanding of multiple dimensions of sustainability. According to this profile, they receive easily understandable ratings for scanned products while shopping. A user study in a real supermarket strengthens the deduced guidelines and indicates that such a system can support customers to make more sustainable product choices.