Software Architecture (SA) assessment provides an analysis of the quality of the high-level view of software-intensive systems, serving as a quality assurance mechanism. Sustainability is a crucial quality for digital ecosystems and as such, it presents assessment challenges due to the multi-dimensional nature of sustainability. This study addresses the challenge of sustainability assessment in SA. Due to a lack of guided sustainability assessment methods, we use an SA evaluation blueprint which is tailored for sustainability assessment. We use a blend of experience-based and quantitative assessment techniques for the assessment of design decision options. The SA assessment is performed on a case study integrating a SaaS-based solution, a learning management system called Canvas, within an educational institute. Our assessment provides an overview of trade-offs between design decision options. We use elements from an open-source toolkit (SAF-toolkit) and a SIS to identify the possible trade-offs and sustainability impacts across different sustainability dimensions. The assessment results identify the trade-offs between QAs and sustainability dimensions (mostly environmental) per design option. This information can help architects make informed decisions about sustainable design choices. Our evaluation method is designed to allow for the assessment of other SAs.