The growing community of CubeSats vendors makes it possible to launch and fly novel payloads for targeted applications by procuring flight-proven CubeSat platforms. The importance of embedded software for such Commercial Off-The-Shelf (COTS) based payload systems has increased to provide more functionality and flexibility. As C OT S components are not designed for space, they warrant extensive software and hardware testing. The ongoing work on how the payload software testing procedure is used in the development of the HYPerspectral Smallsat for Ocean observation (HYPSO1) satellite is presented. This paper discusses the strategy of software development, the challenges that were encountered, and the lessons learned throughout the process. In particular, the advantages of rehearsals, reviews, and manual testing are compared to automated and programmer-driven testing.