I've been working with includesec since 2016 when they were called Pwned Passwords - we worked together closely throughout their existence (until 2018).
They have helped us build out our own infrastructure so that it was able to scale beyond what other vendors could provide at this point during my tenure there! Their support has always taken very seriously but also never let me down or made things difficult along the way which really surprised me given how big some clients can be!! Not much specifically; just make sure everything is documented thoroughly otherwise problems arise later if not now. As mentioned above though, don't skimp here because nothing will go wrong until something does happen after all :) We solved scaling issues by building an entire new platform from scratch using AWS services including Lambda functions etc., solving many different aspects related to high availability/scalability across.