Accessible web interfaces are crucial for the inclusion and independence of all groups in society. Cognitive accessibility aims to make content more accessible for individuals with cognitive disabilities, such as people with learning disabilities (e.g. dyslexia, dysgraphia) and age-related disabilities (e.g. dementia, Alzheimer's disease). It is hard to tell exactly how many people are affected by barriers on the web due to inaccessibility, however, cognitive accessibility is affecting all users on the web to some extent. With an overrepresentation in accessibility research regarding sensory disabilities such as visual or hearing impairments, there is a need for further research on cognitive disabilities. Moreover, there is a lack of research in specific sectors (e.g., commerce, health), solutions anchored in real-world experiences affecting people with cognitive disabilities (PwCDs) and inclusion of PwCDs in the design process for web accessibility. As a contribution, this study aims to investigate the cognitive accessibility in the checkout part of e-commerce websites, adopting a form of a participative design approach. As such, a prototype of a fictive e-commerce website adopting cognitive accessibility was designed along with a counterexample prototype. The prototypes were presented to the target audience in an interaction study, featuring several tasks to evaluate the prototypes. The results showed that the prototype adopting cognitive accessibility was favoured by the users, based on the findings from the collected data. This study concludes that adopting cognitive accessibility to a web interface of the checkout on an e-commerce website is beneficial and perceived as easier to use.