For customers who don't go to great length to protect their own privacy when paying (i.e., all subscribers, I assume) Mullvad should persuade them to replace their subscription with the "bill pay" feature of most checking accounts -- maybe even offer tutorials for common banks. I'm not an expert in the implications of a subpoena and if banks get involved, but it seems like it would at least be a way to keep the revenue stream nearly as healthy (recurring automatically) while also meeting their goal of not maintaining subscription data.