Number one. Having it written in the job quote AND your general terms and conditions. Describe what tools you use and why you use it over a simple installatron WP install. The client signs the job quote, knowing the deeds.
you can describe something like, we use a open source products from Roots. It renders a wordpress website, like you are used to. Only that for us is more efficient, flexible (and maybe cheaper) to roll out fantastic website for you, etc. Please know that this website only runs on this and that.
idk, a quick suggestion.