Yes Fairphone indeed works together with expoena (as do a lot of others). They are a legitimate service provider (more or less - I don’t trust any third party service at all but that’s on me). I too received a confirmation email about a purchase I did over a year ago. In the meantime I did several others so that was confusing for me as well. My guess is that Fairphone is (still) in the process of setting up their shop system (they promised to be ready about a year ago but you can still see that they’re not). Apparently something went wrong or at least not according to plan. I don’t think there’s anything malicious about it though. I’d just ignore it.
Either way, I do support complaining about it directly towards Fairphone as I find it troubling how much Fairphone is outsourcing all of their duties (technical support → zendesk, shop/customer experience → expoena,…)