Thinking about it, it's not really on point for Google™©® to connect you with your customers; your outreach could be considered unsolicited and G could be argued to be enabling spam. What a wonderful situation G has dug for itself here.
The only thought I can come up with is to partially break or otherwise "wait what"-ify your extension's behavior to concretely capture users' attention. You mentioned it does PDF processing of some kind; a relevant example in this case could be injecting new page(s) into generated output (or watermarking, but that may be mechanically trickier, and my knee-jerk connection to watermarking could wind up being "meep, this software has just gone rogue on me" - and depending on my stress level I may not actually read any notices).
Said injected (or overlaid) content would likely immediately first clarify that visiting the options page and clicking a giant green button will 100% revert functionality. It would probably also do well to include a website name (also showing a giant banner), a contact email address (RIP in advance), and possibly a suggestion/invitation to forward the PDF to internal IT for review in case of doubt (which would neatly offload some % of "???" to external labor that is incidentally highly trusted, but may look odd).
FWIW, I noticed the latest versions of Chrome have redesigned the extension UI; overflow icons have moved from the system menu into a dedicated popup. Some users (Windows) likely have this update, while others (eg, myself using Chromium 83.0/Debian, and version-locked enterprise users) don't. So hypothetical screenshots would need to illustrate both flows.
Besides all that, auto-renewing to some (non-low-end, because why not) percentage of the current list price, and straightforwardly handling refunds, seems to be the only obvious fallthrough I can see.
