This study analyzes which factors prompt customers to attribute value to products they design themselves using mass‐customization (MC) toolkits. The assumption that self‐design delivers superior customer value is fundamental to the concept of MC toolkits and can be found in almost any conceptual work in this field. However, spectacular failures reinforce the practical relevance of developing a deeper understanding of why and when MC toolkits generate value for customers—and when they do not. Research to date has assumed that the closer fit between the self‐designed product's characteristics and the preferences of the customer is the dominant source of value. In this research, it is asked whether the enjoyment and perceived effort of the self‐design process have an additional impact on the perceived value of self‐designed products. This question is interesting because one could argue that a rational actor would hardly be willing to pay ex post for an economic good already consumed. The hypotheses are tested on 186 participants designing their own scarves with an MC toolkit. After completing the process, they submitted binding bids for “their” products in Vickrey auctions. Therefore, real buying behavior, not merely stated intentions, is observed. The present study finds that the subjective value of a self‐designed product (i.e., one's bid in the course of the auction) is indeed impacted not only by the preference fit the customer expects it to deliver but also by (1) the process enjoyment the customer reports, (2) the interaction of preference fit and process enjoyment, and (3) the interaction of preference fit and perceived process effort. In addition to its main effect, preference fit can be interpreted as a moderator of the value‐generating effect of process evaluation: in cases where the outcome of the process is perceived as positive (high preference fit), the customer also interprets process effort as a positive accomplishment, and this positive effect adds (further) value to the product. It appears that the perception of the self‐design process as a good or bad experience is partly constructed on the basis of the outcome of the process. In the opposite case (low preference fit), effort creates a negative effect that further reduces the subjective value of the product. Likewise, process enjoyment is amplified by preference fit, although enjoyment also has a significant main effect, which means that regardless of the outcome, customers attribute higher value to a self‐designed product if they enjoy the process. In a way, this effect resembles of the classic story of Tom Sawyer and the fence, in which Tom manages to “frame” the tedious chore of whitewashing a fence as a rare opportunity—thus persuading his friends to pay him for letting them work. Manufacturers designing an MC system therefore are advised to designing MC toolkits in a way that they elicit positive affective reactions that make their customers value their work.