First you will need to define your target group. What type of merchants are you targeting? What systems do they use? How do you track the systems? How do you ensure you teams are trained on the systems so they can talk to the merchants the right way? How do you keep it updated? What System do you integrate first? What features are possible on the different Systems. Ideally everyone in the company will have to be aligned to ensure less overhead confusion. Often the integration roadmap is decided by the Salesperson who yells the loudest or the typical system that the first merchant requests.